Hyssop

What is Hyssop?

A 26-year-old woman had a seizure on the second of two days taking 10 drops of hyssop essential oil, a case reported by French toxicologists Millet and colleagues in 1981. The same plant is sold as tea, and the retail tea page checked for this issue carried no warning at all. That gap between the oil and the leaf is the subject of this issue.

Hyssopus officinalis is a perennial, woody-based subshrub of the mint family, Lamiaceae, growing 30 to 70 cm tall, with narrow, pointed, strongly aromatic leaves and spikes of purple-blue, pink or white flowers from June to October. A 2026 review in Plants (Nurzyńska-Wierdak) counts about 10 to 12 species in the genus and places H. officinalis natively in southern Europe, the Middle East and the Caspian region. It is grown or wild-collected in at least 19 countries, from France, Germany and Hungary to Iran, China, India and the United States, and named cultivars include Sophie, Erfurter Ysop, Blankyt and Perlay. A plantation stays productive for about 10 years, yields 0.67 to 3.26 tonnes of dried herb per hectare, and gives 10 to 20 kg of distilled oil per hectare. Beekeepers value it too: the same review puts its honey productivity at 200 to 400 kg per hectare. The herbal drug is the dried flowering tops and leaves, harvested in full bloom, when oil content peaks.

The plant's chemistry varies more than its appearance. Most commercial hyssop oil is dominated by two closely related ketones, pinocamphone and isopinocamphone, but the same review cites a cluster analysis identifying at least eight chemotypes, one of them a Nigerian oil that was 70.9% α-pinene, and a linalool-rich form is sold as var. decumbens. A bottle labeled only "Hyssopus officinalis" does not say which one is inside.

Common Names

Hyssop, common hyssop, herb hyssop, blue hyssop; Ysop (German); hysope (French); issopo (Italian); hisopo (Spanish); zufa (Persian and Uyghur medicine, where the name also covers the related Hyssopus cuspidatus). Several unrelated plants borrow the name: anise hyssop and giant hyssop (Agastache species), water hyssop (Bacopa monnieri) and the biblical "hyssop," most likely a wild oregano (see Special Considerations).

Primary Active Compounds

The essential oil makes up 0.3% to 1.8% of the dried herb in most analyses, and 1.38% in fresh flowers against 0.75% in fresh leaves in one Iranian comparison (Moghtader, as reviewed in 2026). In typical European and Asian material the oil is 60% to 75% pinocamphone plus isopinocamphone: Aćimović and colleagues measured 61.1% to 74.4% combined across white, pink and purple-flowered Serbian plants, and Németh-Zámbori and colleagues found 70.7% in the German cultivar Erfurter Ysop. β-pinene usually comes third, at roughly 5% to 20%, with smaller amounts of pinocarvone, myrtenol, germacrene D, elemol and bicyclogermacrene. Outside the oil, the leaves carry rosmarinic and caffeic acids (present in every Hungarian strain Varga and colleagues tested in 1998), the flavonoids diosmin, linarin, luteolin, quercetin and apigenin, two α-glucosidase-inhibiting syringoylglycerol glucosides isolated by Matsuura and colleagues (Phytochemistry, 2004), and a water-soluble polysaccharide labeled MAR-10.

Key Note

The two ketones are the core of the safety story. In mice they act as GABA-A receptor antagonists, blocking the brain's main inhibitory signal, and they are the reason hyssop appears on the convulsant oil lists compiled by Burkhard and colleagues (Journal of Neurology, 1999) and by Tisserand and Young (Essential Oil Safety, 2nd edition, 2014). The leaf contains them too, but in amounts that depend on chemotype, harvest and how the tea is made, and no measurement of them in a brewed cup was located for this issue.

Hyssop is a kitchen and liqueur herb with one small controlled asthma trial, one cosmetic skin study, and a concentrated oil that has caused seizures at drop-level doses.

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Primary Functions & Benefits

Hyssop's reputation is respiratory first and digestive second. The Persian physician Avicenna (980 to 1037) wrote in the Canon of Medicine that its most common use was for lung complaints including asthma, as the authors of the one modern trial note, and the 2026 Plants review lists hyssop herb as official in the pharmacopoeias of France, Portugal, Romania, Sweden and Germany, with expectorant, gastric and diuretic indications. Germany's Commission E took the opposite view of the evidence and issued a negative monograph, finding its efficacy not proven.

Asthma: the one controlled trial

Daneshfard and colleagues (Tanaffos, 2024; registered as IRCT20110811007297N5) enrolled 70 adults with mild to moderate persistent asthma at Shiraz University of Medical Sciences, Iran, and randomly assigned them to 5 mL of hyssop syrup or a sugar syrup twice daily for 4 weeks, on top of their usual inhaled corticosteroid and long-acting bronchodilator. Each 5 mL represented 6 g of dried flowering herb, steeped as a 4-hour hot water infusion and concentrated into a 50% sugar base, so the daily intake was the extract of 12 g of herb. Sixty patients completed the study, 30 per group. In the hyssop group, 4 patients withdrew because their cough worsened and 1 because breathlessness increased.

The trial did not report a simple hyssop-versus-placebo comparison for all 60. The authors split patients after the fact into productive (wet) cough and dry cough, a distinction drawn from Persian medicine. In the 16 hyssop patients with productive cough, FEV1 (the volume blown out in 1 second, as a percentage of predicted) rose by a mean of 20.43 points against 0.28 in the 14 matching placebo patients, and Asthma Control Test scores rose by 9.56 points against 0.50 at week 4. In the 14 hyssop patients with dry cough, FEV1 fell by 8.00 points against a 0.93-point rise on placebo, and wheezing worsened. The five withdrawals for worsening symptoms were all dry-cough patients. The authors concluded that hyssop syrup suits asthmatics with productive cough and is "not suitable" for those with a dry cough. A post hoc split of 30 people into groups of 14 to 16 is a hypothesis, not a finding.

Skin barrier: one cosmetic study

Tito and colleagues (Cells, 2026), working largely at the Italian cosmetic biotechnology company Arterra Bioscience, tested an extract made from hyssop cell cultures rather than the field-grown plant. Applied at 0.5% to mechanically stressed skin in 10 volunteers, it raised activity of the ceramide-processing enzyme β-glucocerebrosidase by 30% and cut transepidermal water loss by 15%, while the placebo area lost 55% of its enzyme activity and gained 25% in water loss. Ten people, a company-made extract and a cosmetic endpoint say nothing about tea or oil taken by mouth.

Blood sugar, in rodents

A 2003 mouse study by Miyazaki's group (J Nutr Sci Vitaminol) found that an oral hyssop leaf extract given before a sugar load blunted the early rise in blood glucose in mice: 300 mg per kg before sucrose and 100 mg per kg before maltose, within 30 to 60 minutes. In isolated rat gut, the extract at 0.5 and 1.0 mg per mL slowed glucose release from sucrose and maltose but not absorption of glucose itself, which points to inhibition of the intestinal enzyme α-glucosidase, the same target as the prescription drug acarbose. No human glucose study was located.

Gut spasm, in isolated tissue

Hyssop oil relaxes intestinal smooth muscle in the dish. Mazzanti and colleagues reported that it inhibited contractions caused by acetylcholine and barium chloride at IC50 values of 37 and 60 micrograms per mL, and a guinea pig ileum study by Lu and colleagues attributed much of the effect to isopinocamphone (IC50 values of 61.9 and 70.4 micrograms per mL against the same two triggers, versus 42.4 and 48.3 for the whole oil), as summarized in the 2026 Plants review. This is the laboratory basis for the "carminative" and "antispasmodic" label claims.

Seizures in rats: the extract runs the other way

Water and water-alcohol extracts of the leaf, which carry the polyphenols and little of the oil, behaved as anticonvulsants in rats. Gholami and colleagues (Avicenna Journal of Phytomedicine, 2020) injected an aqueous extract at 50, 100 or 200 mg per kg before the convulsant drug pentylenetetrazole; only 100 mg per kg significantly delayed the most severe seizure stages. Fatahinezhad and colleagues (Neurochemical Research, 2022) gave a hydroalcoholic extract at 25, 50 and 100 mg per kg for 14 days and found a higher tonic seizure threshold and lower mortality, an effect the benzodiazepine blocker flumazenil reduced. These results concern the extract, not the oil, and they are no reason for anyone with epilepsy to use hyssop.

Antimicrobial activity

The oil is a moderate antimicrobial at best: Hristova and colleagues found it inhibited 52 Candida strains, including fluconazole-resistant ones, but Knezevic and colleagues (Journal of Pharmaceutical and Biomedical Analysis, 2018) found it inactive against Helicobacter pylori at the highest concentration tested, 4 microliters per mL. Crude leaf extracts and the polysaccharide MAR-10 inhibited HIV in cell culture in 1990 and 1995, which is a dish result, not evidence for any antiviral use.

Forms & Standardization

No hyssop product on the US market is standardized to a marker compound, and the one controlled trial used a homemade-style infusion syrup rather than anything sold. What separates the forms is how much essential oil, and which chemotype of it, each one carries.

Dried herb and tea bags

Cut and sifted hyssop is sold in bulk (Frontier Co-op lists an organic 1 lb bag) and in tea bags. Buddha Teas' Hyssop Tea is a typical retail example: 24 g net weight, 1.33 g of organic hyssop herb per bag, with directions to use 1 bag per cup, heat water to 205 °F and steep 3 to 6 minutes. The oil content of the leaf depends on when and what was harvested. Can and colleagues measured 0.93% at full flowering, the seasonal peak; Yousefzadeh and Naghdi Badi found full-flowering oil yields about five times those before flowering; and Németh-Zámbori and colleagues measured 1.401% in first-year plants, 1.022% in second-year and 0.880% in third-year plants (all as reviewed in Plants, 2026). The tea labels checked for this issue did not state harvest stage, plant age or oil content.

Essential oil, pinocamphone type

This is the oil most suppliers sell as plain "hyssop." Representative analyses of steam-distilled or hydrodistilled oil:

• Turkey (Can and colleagues): pinocamphone 38.41% to 41.85%, isopinocamphone 22.73% to 22.99%, β-pinene 7.92% to 8.94%.

• Serbia, three flower colors (Aćimović and colleagues): combined pinocamphones 61.1% to 74.4%.

• Iran, the trial's own plant material (Daneshfard and colleagues, 2024): cis-pinocamphone 41.60%, trans-pinocamphone 32.76%, β-pinene 15.24%.

• India, Western Himalaya (Pandey and colleagues): cis-pinocamphone 49.7% to 57.7% in leaf, flower and stem oils, with pinocarvone 5.5% to 24.9%.

• Tisserand and Young's reference range: pinocamphone 31.2% to 42.7% and isopinocamphone 30.9% to 39.2%, and their dermal limit is based on an oil with 82.3% pinocamphone.

Papers label the two isomers inconsistently. Some report "pinocamphone" and "isopinocamphone," others "trans-" and "cis-pinocamphone," and a few mix the conventions within one table, so the combined total is the safest figure to compare between products.

Essential oil, linalool type (var. decumbens)

A low-growing form sold as Hyssopus officinalis var. decumbens (decumbens means lying down) gives an oil of roughly 50% linalool with only 0.5% to 1.0% pinocamphone, according to Tisserand and Young, who judge its seizure risk "probably negligible." A hyssop from Banon in the French Alps, cited in the 2022 review by Sharifi-Rad and colleagues, fits the same pattern: 49.6% linalool, 13.3% 1,8-cineole and only minor ketones. Eden Botanicals sells a Spanish steam-distilled "Hyssop Decumbens EO" (5 mL for $8 on the day checked) and cites Tisserand and Young, page 307, for "None known" under safety, while its own page warns that common hyssop "may contain up to 80% of the ketone pinocamphone."

Essential oil, 1,8-cineole and other types

Spanish oils can be dominated by 1,8-cineole: 52.89% in one analysis cited by Sharifi-Rad and colleagues, and 53% in an industrial distillate studied by Ortiz de Elguea-Culebras and colleagues (Chemistry & Biodiversity, 2017). An Iranian white-flowered variety yielded oil with 31.85% camphor alongside 30.11% cis-pinocamphone (Mohamadpoor and colleagues, as reviewed in 2026). Essentially You Oils of Ottawa lists 1,8-cineole at 52.40% for its organic Spanish hyssop oil, names pinocamphone and isopinocamphone among the major constituents without percentages, and labels the oil for external use only.

Extracts, tinctures and syrups

Water extracts are dominated by the polyphenols, but hyssop-specific data on how much oil a water or alcohol extract carries were not located. Commercial tinctures rarely state a herb-to-solvent ratio, and none located declared a ketone content. Supercritical carbon dioxide extraction gave a more uniform oil profile than steam distillation in a 1998 Hungarian comparison (Varga and colleagues). In China, a 2026 quality-control study (Zhao and colleagues, Molecules) proposed HPLC assay of diosmin, linarin and rosmarinic acid as markers for Herba Hyssopi, the Uyghur medicine made from Hyssopus cuspidatus, which shows what a marker specification for hyssop could look like.

What the studies used

The oral asthma trial used a 4-hour hot water infusion of Iranian herb whose distilled oil was 74% pinocamphones, concentrated into syrup at 6 g of herb per 5 mL. The skin study used a cell-culture extract at 0.5%. The rodent glucose and seizure studies used methanol, water or water-alcohol leaf extracts. No controlled human study of hyssop oil taken by mouth was located, and no product found on sale matches the trial syrup.

Food Sources

Hyssop is itself a food herb rather than a nutrient found in other foods, so this section covers how people are exposed to it and, above all, to its ketones.

As a culinary herb and flavoring

The leaves season soups, broths, salads and spice mixes, and the plant flavors liqueurs; bees work its flowers for honey. In the United States hyssop appears twice on the FDA's generally-recognized-as-safe lists: as a spice under 21 CFR 182.10 and as an essential oil or natural extractive under 21 CFR 182.20, without a numeric limit in either. In the European Union, pinocamphone is not among the substances in Annex III of the flavorings regulation (EC) No 1334/2008, the list that restricts thujone, pulegone, estragole and similar natural constituents, so no EU maximum level for pinocamphone in food was located. Commission E's negative monograph (Bundesanzeiger No. 162, August 29, 1992) raised no objection to hyssop herb as a flavoring in tea blends at up to 5%.

What a cup of hyssop tea could contain

No published measurement of pinocamphone or isopinocamphone in a brewed cup of hyssop tea was located, so the following is arithmetic, not data. One 1.33 g Buddha Teas bag at the leaf's typical 0.3% to 1.8% oil content holds roughly 4 to 24 mg of oil. At 60% to 75% ketones, that is about 2.4 to 18 mg of ketones in the leaf before any water touches it. The fraction that a 3 to 6 minute steep pulls out has not been measured for hyssop. The nearest measured analogue is sage: Walch and colleagues (Chemistry Central Journal, 2011) found that hot water recovered on average 30% (range 14% to 48%) of the thujone that 60% ethanol extracted from the same leaves. If hyssop behaved similarly, a cup would carry roughly 1 to 5 mg of pinocamphones. Tisserand and Young's oral limit of 0.1 mg per kg, set for the two ketones, works out to 7 mg for a 70 kg adult, so a few strong cups from a high-oil batch could plausibly reach it, while a single ordinary cup probably would not. For a 20 kg child the same limit is 2 mg, inside the modeled range for one cup. That conclusion rests on two assumptions nobody has tested for this plant.

The oil, for comparison

Ten drops of oil, at the common pharmacy convention of 20 drops per mL, is about 0.5 mL of liquid. At 60% to 75% ketones, that is on the order of 300 mg of pinocamphones, which is roughly 60 to 300 times the modeled cup and more than 40 times Tisserand and Young's limit for a 70 kg adult. Ten drops on each of two days preceded the 1981 seizure.

The trial syrup

The Iranian asthma syrup supplied the extract of 12 g of herb a day. By the trial's own measurement, that herb yielded 0.4 mL of oil per 100 g, so the daily herb contained about 0.048 mL of oil, roughly three quarters of it pinocamphones, before a 4-hour hot water steep. How much of that reached the syrup was not measured.

Cooking and processing

Harvest stage changes the ketone mix: the 2026 Plants review records that trans-pinocamphone dominates oil from plants still in vegetative growth while cis-pinocamphone rises as they flower. No study located measured ketone loss during cooking. Hyssop has no nutritional role that a varied diet needs to fill.

Who Should Take Hyssop

No group has been shown in an adequate trial to benefit from hyssop, and no one should swallow hyssop essential oil. The realistic users are:

• Adults who enjoy it as a culinary herb or an occasional cup of tea, understanding that the respiratory and digestive claims on tea boxes rest on tradition and the Commission E verdict was negative.

• Adults with asthma and a productive cough who are curious about the Iranian syrup result. That finding came from a post hoc subgroup of 16 people, the same syrup made dry-cough patients worse, and it was an add-on to inhaled medicine, never a replacement. Any trial of it belongs with the prescribing clinician.

• Aromatherapy users who want the scent, who should choose a labeled linalool (var. decumbens) oil and use it only diluted on skin or in a diffuser.

Hyssop treats no asthma attack, infection or high blood sugar.

Who Should AVOID or Use Caution

Contraindications

• Anyone, for swallowing hyssop essential oil of any chemotype. The seizure reports involved amounts from 10 drops to about half a coffee spoon, taken by mouth, and no oral amount has been shown to be safe.

• Epilepsy or any past seizure. The Iranian trial excluded people with a seizure history, and the oil's ketones block the same GABA-A receptor that several anti-seizure drugs work through.

• Pregnancy and breastfeeding. Drugs.com's professional monograph advises avoiding hyssop in both, citing emmenagogue (menstrual-stimulating) and abortifacient effects. Reproductive toxicity has never been tested in people or animals as far as this issue could find, so nobody has shown the tea to be low-risk in pregnancy. Avoid it.

• Children. A 6-year-old who had frequently been given 2 to 3 drops for asthma convulsed after about half a coffee spoon of oil (Arditti, 1978). Never give hyssop oil to a child by mouth, do not apply pinocamphone-type oil to a child's skin at any age, and keep every hyssop oil bottle out of reach.

• Known allergy to hyssop or other mint-family plants. The trial excluded people allergic to pennyroyal, peppermint, thyme, hyssop, lavender or savory.

Use Caution

• Asthma with a dry, non-productive cough. In the only trial, this subgroup's lung function fell by 8 points of predicted FEV1, and all five withdrawals for worsening cough or breathlessness came from it.

• Medicines that lower the seizure threshold, such as bupropion, tramadol and theophylline. The trial excluded theophylline users. Combining any of these with hyssop oil, or with large amounts of strong tea, stacks two seizure risks.

• Diabetes treated with insulin or a sulfonylurea such as glipizide. Hyssop extract slowed sugar digestion in rodents, and the trial excluded people with diabetes, so no human data exist either way.

• Anyone using several aromatherapy oils at once. Sage, rosemary, wormwood and other ketone-rich oils add to hyssop's load.

Critical Safety Point

Hyssop essential oil has caused convulsions in a child and two young adults at doses measured in drops, and in rats repeated small doses that caused no seizure on day 1 produced electrical seizures after 3 to 4 days. Never take it by mouth, never give it to a child, and treat any twitching, confusion or loss of consciousness after exposure as an emergency.

Recommended Dosages

No dose of hyssop has been established by a regulator or by an adequately sized trial, and there is no oral dose of hyssop essential oil at all.

Dried herb and tea

Commission E's negative monograph gives no therapeutic dose. Retail tea directions are culinary rather than clinical: Buddha Teas specifies one 1.33 g bag per cup with no daily maximum and no pregnancy warning on its product page. Given the unmeasured ketone content of the cup (see Food Sources), the cautious reading for a healthy, non-pregnant adult is an occasional cup of a standard-strength infusion, not several strong cups a day for weeks. The amount used in the Iranian trial was a supervised research dose for asthma and is described under Benefits, not here.

Essential oil, by mouth

None. Tisserand and Young's oral limit of 0.1 mg per kg of body weight for the pinocamphone isomers is a ceiling on exposure to those ketones, not an amount to take, and it equals a fraction of a single drop for an adult.

Essential oil, on skin

For pinocamphone-type oil, Tisserand and Young set a maximum dermal concentration of 0.3%, which is less than one drop per 10 mL of carrier oil. Their equivalent limit for the ketones themselves is 0.25%. For a labeled linalool-type (var. decumbens) oil they list no specific limit, so ordinary aromatherapy dilutions apply. Do not apply pinocamphone-type oil to children at any age.

Duration

No duration has been studied beyond the trial's 4 weeks. Because the rat data show a cumulative effect of repeated small doses of the oil, there is no case for daily long-term use of any concentrated hyssop preparation.

Timing & Administration

Hyssop tea is traditionally taken after meals for digestion or between meals for coughs; neither timing has been tested. The trial syrup was taken before breakfast and at bedtime. Steeping time matters: in the sage analysis cited above, ketone extraction rose roughly linearly for about 5 minutes and then leveled off, so a steep at the short end of the 3 to 6 minute range printed on hyssop tea boxes should extract less. Diffusing oil in a ventilated room avoids the oral route entirely; there is no reason to swallow it, add it to drinks or put it in the mouth.

Timeline of Effects

The only human timeline comes from the 4-week asthma trial; everything else is either a rodent measurement or a toxic onset.

Asthma syrup (Daneshfard, 2024)

Lung function and Asthma Control Test scores were measured at baseline and at week 4, with a telephone follow-up at week 8, four weeks after the syrup stopped. At week 8 the productive-cough subgroup showed no significant difference from placebo in its control-score change (0.31 against minus 0.78 points, p = 0.75); the paper does not state clearly whether that figure is measured from baseline or from week 4, so whether the week 4 gain persisted cannot be read from it. In the dry-cough subgroup, control scores improved significantly between week 4 and week 8, after the syrup had stopped, which the authors took as recovery from hyssop's effect. Four weeks was the full length of exposure tested.

Blood sugar, in mice

The glucose-blunting effect appeared within 30 to 60 minutes of a single oral extract dose given before the sugar load, consistent with an enzyme-blocking action in the gut rather than a lasting metabolic change. No repeated-dose or human data exist.

Toxic onset, which is the timeline that matters most

The 18-year-old in the 1978 report lost consciousness about 1 hour after 30 drops. The 26-year-old seized on the second day of taking 10 drops daily, which fits the cumulative pattern Millet's group saw in rats given repeated subclinical injections. The 6-year-old had been given 2 to 3 drops repeatedly before the larger dose that preceded his convulsion. A clean first day with hyssop oil is not evidence that the next day will be clean.

When to stop and reassess

For hyssop tea taken for digestion or cough, a few days is a fair personal trial for an acute complaint. A cough that persists beyond 3 weeks, or any wheeze, breathlessness or blood in the sputum, needs medical assessment, and an asthmatic whose cough or breathing worsens on hyssop should stop it, as five trial participants did.

Benefits of Taking Hyssop

The documented benefits are few and small:

• In the one controlled trial, asthmatics with a productive cough improved on the syrup over 4 weeks, but only in a subgroup of 16 defined after the results were in.

• A 0.5% cell-culture extract improved two skin-barrier measures in 10 volunteers, in a company-led cosmetic study.

• Leaf extracts blunted the after-meal sugar spike in mice and calmed drug-induced seizures in rats, results that justify human studies and do not substitute for them.

• The oil relaxes gut muscle and inhibits Candida in the dish, at concentrations not shown to be reached in the body.

• As a flavoring and tea, hyssop is a pleasant, inexpensive, GRAS-listed culinary herb.

The pharmacopoeial listings in five European countries and the Avicennan tradition record long use for coughs and digestion. They are history, and Commission E did not find them sufficient to support efficacy.

Potential Negatives & Side Effects

In the trial

The hyssop group reported thirst, dry mouth and throat irritation, and 5 of 35 assigned to hyssop stopped because cough or breathlessness worsened, all from the dry-cough subgroup. The placebo group's 5 dropouts were for dissatisfaction (4) and moving away (1), not symptoms. The paper gives no adverse event counts by type.

From the oil

The serious harm on record is convulsions, covered under Toxicity Symptoms. Drugs.com's professional monograph states that hyssop oil was nonirritating to skin in animal and human testing, but skin is not a barrier to the ketones: the 2022 Sharifi-Rad review states that pinocamphone has produced seizures when used both internally and topically. Mint-family allergy is possible with any form.

Deficiency Symptoms

What hyssop addresses

No deficiency state exists. Hyssop supplies no nutrient the body needs from it, and its polyphenols are shared with rosemary, thyme and other everyday kitchen herbs.

Bottom line

There is nothing to replete, no blood test that could show a need, and no symptom of going without it.

Toxicity Symptoms

At high intake

Toxicity is a property of the oil and its two ketones. The human record is small and old but consistent:

• A 6-year-old boy, regularly given 2 to 3 drops for asthma, convulsed after about half a coffee spoon (perhaps 1 mL) during a severe attack and recovered after 3 days in hospital (Arditti and colleagues, 1978, as compiled by Tisserand and Young).

• An 18-year-old woman took 30 drops for a cold and about an hour later lost consciousness for 10 minutes, with generalized contractions and a bitten tongue (Arditti and colleagues, 1978).

• A 26-year-old woman took 10 drops on each of two consecutive days and had a seizure on the second (Millet and colleagues, 1981).

Millet and colleagues (Clinical Toxicology, 1981, volume 18, pages 1485 to 1498) described eight human intoxications from commercial essences of sage, hyssop, thuja and cedar, with tonic-clonic convulsions as the main symptom, and then tested the oils in unanesthetized rats with electrodes recording brain activity. Hyssop oil was the more potent convulsant of the two oils compared: brain-wave changes stayed subclinical up to 0.08 g per kg, convulsions appeared above 0.13 g per kg, and they became lethal above 1.25 g per kg, against 0.3, 0.5 and 3.2 g per kg for sage oil. The group's 1979 French paper found isolated pinocamphone lethal to rats above 0.05 mL per kg. Höld and colleagues (Xenobiotica, 2002) gave mice the two pinanone ketones or hyssop oil by injection and recorded tonic and clonic convulsions, with LD50 values of 175 to more than 250 mg per kg, which diazepam alleviated. All of these animal doses were injected into the abdominal cavity and are not human dosing data.

Signs to reduce or stop

• Muscle twitching, jerking or unusual restlessness after using hyssop oil by any route.

• Confusion, drowsiness, a staring spell or loss of consciousness, which need emergency care; in the US, Poison Help is 1-800-222-1222.

• Worsening cough, wheeze or breathlessness in an asthmatic taking hyssop in any form.

• Dry mouth, thirst or throat irritation from tea or syrup, the adverse effects reported in the trial.

• Rash or itching where diluted oil was applied.

General note

Burkhard and colleagues (Journal of Neurology, 1999) reported three previously healthy patients with oil-related seizures and listed hyssop among 11 plants whose oils are powerful convulsants; the abstract does not say which oils their patients used. No poisoning, liver injury or death attributed to hyssop tea was located. That absence reflects how little the tea has been studied as much as it reflects safety.

How Hyssop Works

Hyssop's effects come from two unrelated groups of compounds that push in opposite directions: the volatile ketones in the oil, and the water-soluble polyphenols and glycosides in the leaf.

The ketones block inhibition in the brain

Pinocamphone and isopinocamphone are the two stereoisomers of 3-pinanone, bicyclic monoterpene ketones made in the plant by oxidation of β-pinene. Höld and colleagues at the University of California, Berkeley (Xenobiotica, 2002) showed that both isomers, and whole hyssop oil, displaced a radiolabeled probe ([3H]EBOB) from the chloride channel of the GABA-A receptor in mouse brain membranes, with IC50 values of 35 to 64 micromolar. GABA-A is the receptor that lets nerve cells damp each other's firing; blocking its channel removes the brake, which is how convulsants of this class work. The convulsions their mice developed were alleviated by diazepam, a drug that strengthens the same receptor.

The body detoxifies them, slowly

In mouse and human liver microsomes and with recombinant human CYP3A4, cis-3-pinanone was converted mainly to 2-hydroxy-cis-3-pinanone, which was less toxic and a weaker GABA-A blocker; the same metabolite was the main one found in the brains of injected mice. The trans isomer was metabolized more slowly than the cis isomer in every system tested. Mouse urine after oral dosing contained conjugated metabolites. Millet's rat experiments, in which repeated subclinical doses produced seizure activity after several days, fit a compound that accumulates or sensitizes faster than it is cleared, though no human pharmacokinetic data exist.

The oil relaxes smooth muscle

On isolated guinea pig and rabbit intestine, hyssop oil and isopinocamphone inhibited contractions triggered by acetylcholine and barium chloride, with the whole oil more potent than the single ketone. The asthma trial's authors proposed a similar anticholinergic action on airway muscle, involving β-pinene and cis-pinocamphone, to explain the productive-cough result; they did not test it.

Leaf glycosides slow sugar digestion

Matsuura and colleagues isolated two syringoylglycerol glucosides from dried hyssop leaves that inhibit rat intestinal α-glucosidase, the enzyme that splits sucrose and maltose. Because the extract slowed glucose release from those sugars but not glucose absorption itself, the mouse glucose effect is best explained at the gut wall.

Leaf polyphenols calm cells, and possibly the brain

Rosmarinic acid, diosmin and linarin, the markers proposed for Herba Hyssopi, suppressed nitric oxide and inflammatory cytokine release in cultured cells (Zhao and colleagues, 2026). In the rat kindling study, flumazenil, which blocks the benzodiazepine site of the GABA-A receptor, reduced the extract's anticonvulsant effect, suggesting the leaf extract enhances the very receptor the oil's ketones block. Which effect wins in a cup of tea has not been studied.

Synergistic Supplements

No controlled trial of a combination including hyssop was located. An uncontrolled 2007 study in Hamdard Medicus gave 25 people with mild to moderate asthma a three-herb formula of Adhatoda vasica, licorice and hyssop for 6 weeks and reported less frequent breathlessness and better peak flow, with no comparison group reported. In horses, an herbal composite of garlic, white horehound, boneset, aniseed, fennel, licorice, thyme and hyssop produced no statistically significant change in a 6-horse crossover pilot at the Ontario Veterinary College, published in 2007. Stacking it with rosemary, thyme or lemon balm tea adds more of the same rosmarinic acid rather than a new action. The combinations to avoid are with other ketone-rich essential oils, which stack more convulsant ketones without any shown benefit.

Interactions & What NOT to Take

This issue located neither a human interaction study nor an interaction case report involving hyssop; Drugs.com describes interactions as "none well documented." The cautions below are inferences from the pharmacology, labeled as such.

Anti-seizure medicines

People taking clonazepam, clobazam, phenobarbital or any other anti-seizure drug should not use hyssop oil. The oil's ketones block the GABA-A channel these drugs depend on, and an oil that lowers the seizure threshold works directly against the treatment. Hyssop's place on published convulsant lists is reason enough, even without an interaction study.

Drugs that lower the seizure threshold

Bupropion, tramadol and theophylline each carry seizure warnings of their own. Adding a convulsant oil compounds the risk. The asthma trial excluded theophylline users, though it did not say why.

Strong CYP3A4 inhibitors (inference)

Human CYP3A4 converts cis-3-pinanone to a less toxic metabolite (Höld and colleagues, 2002). In principle, ketoconazole, itraconazole, clarithromycin or ritonavir, which block CYP3A4, could slow that detoxification and raise ketone exposure from hyssop oil. This has not been tested; it is one more reason the oil should not be swallowed by anyone on these drugs.

Glucose-lowering drugs (inference)

Hyssop leaf extract inhibited intestinal α-glucosidase in rodents, the target of acarbose and miglitol. Combining a concentrated extract with those drugs, insulin or a sulfonylurea could add to their effect. Ordinary tea quantities are unlikely to matter, but no human data exist.

Other essential oils

Do not combine hyssop oil with sage, thuja, wormwood, tansy, pennyroyal or camphor-rich oils such as some rosemary oils. Their ketones (thujone, camphor, pulegone and others) appear on the same convulsant lists and add to the total load.

Quality, Testing & Adulteration

The chemotype problem

The most important quality fact about hyssop oil is one the front label usually omits: which chemotype it is. Two bottles both labeled "Hyssopus officinalis" can hold an oil that is three quarters convulsant ketones or one that is half linalool with under 1% ketone. Essentially You Oils' product sheet illustrates the confusion: it reports 52.40% 1,8-cineole for its Spanish oil, yet warns that "due to the presence of pinocamphon" a physician should be consulted and that this type "should not be used in aromatherapeutic applications," without giving the ketone percentage that would let a buyer judge. Seed lots are mixed too; the 2026 Plants review notes that uniform plant populations are hard to obtain and that raw material on the market often varies in composition.

Name confusion

Anise hyssop (Agastache foeniculum) is a different genus with an anise-scented oil. Giant hyssop is a name for other Agastache species, and "giant-hyssop herb" appears in English translations of Chinese herbal formulas. Water hyssop is Bacopa monnieri, the memory herb sold as brahmi. None of these is Hyssopus, and none should be assumed to share its chemistry or its hazards.

Substitution in Asian trade

In Uyghur medicine the official Herba Hyssopi is Hyssopus cuspidatus. Zhao and colleagues (Molecules, 2026) reported that cultivated H. officinalis is frequently used as a substitute and that a separate adulterant species also circulates, and they developed chemical markers (quercetin, diosmin, linarin and lonicerin among them) that separated authentic material from both. Here, ordinary hyssop is the substitute.

Third-party testing

No USP monograph, NSF certification program or ConsumerLab review covering hyssop herb or oil was located. In the European Union the herb has no HMPC monograph to test against. Quality therefore depends entirely on what the seller chooses to measure and disclose.

What a useful certificate shows

For oil: the full botanical name including variety (for example var. decumbens), country of origin, plant part, distillation method and date, and a GC or GC-MS profile giving percentages for pinocamphone, isopinocamphone, linalool, 1,8-cineole, β-pinene and camphor, plus a statement that it is not for internal use. For herb or tea: Hyssopus officinalis named in Latin, the flowering aerial parts, harvest year, a volatile oil content by distillation, and results for lead, cadmium, arsenic, mercury, pesticides and microbial load against stated limits.

What you can check yourself

Read the botanical name: "var. decumbens" or "linalool" on the label points to the low-ketone oil, and a plain species name without a GC report should be treated as the pinocamphone type. Smell is a rough guide: the ketone-rich oil is sharply camphoraceous, while the linalool type is softer and floral. Check whether a tea lists health claims without warnings; Buddha Teas' product page claims hyssop "Promotes Respiratory Health" and "Reduces Inflammation" yet carries no pregnancy caution. Any hyssop oil sold with directions to swallow it, or packaged as a dietary supplement for internal use, should not be bought.

Special Considerations

Which plant is the biblical hyssop?

Probably not this one. The Hebrew Bible's ezov is the bundle used to mark doorposts with blood at the first Passover (Exodus 12:22), appears with cedar wood in the purification rites of Leviticus 14 and Numbers 19, is invoked in Psalm 51:7 ("purge me with hyssop"), and in 1 Kings 4:33 is the small plant "that springeth out of the wall." Greek translators rendered ezov as hyssopos, and the name was later attached to the European herb; the 2026 Plants review describes the genus name as arising from a misinterpretation of the Hebrew word. Old Dominion University's Bible plants project identifies the most likely candidate as Origanum syriacum, known in English as Syrian hyssop and in Arabic as one of the za'atar herbs, which grows most often on rocky ledges and outcrops, and notes that some writers have proposed caper (Capparis spinosa) instead. The New Testament use, a sponge of sour wine raised on hyssop at the crucifixion (John 19:29), has prompted debate about how a small herb could hold up a sponge. None of this bears on safety, except that "biblical hyssop" marketing describes a plant that may never have been Hyssopus officinalis.

Persian and Uyghur medicine

The Iranian trial was designed around Persian medicine's division of lung disease into "wet" and "dry" temperaments, and its authors describe it as the first trial of hyssop to take that distinction into account. In Xinjiang, zufa is a principal cough and asthma remedy, but the official species there is H. cuspidatus, and its chemistry (rosmarinic acid and eucalyptol are its proposed quality markers) differs from European H. officinalis.

Regulatory status

• United States: GRAS as a spice (21 CFR 182.10) and as an essential oil or extractive (21 CFR 182.20); sold as a dietary supplement and tea without FDA review of safety or efficacy.

• European Union: no hyssop entry appears on the HMPC priority lists of July 2024 or July 2026, so there is no EU herbal monograph or public statement for the herb or the oil. The committee has issued public statements on other essential oils, including bergamot oil (2012), which makes hyssop's absence a gap rather than a verdict.

• France: Article D4211-13 of the Public Health Code (current version in force since August 8, 2007) reserves retail sale of hyssop essential oil to pharmacists.

• Germany: Commission E negative monograph (1992).

Research Status & Evidence Quality

Tier 1, oil chemistry (strongest)

Many GC-MS analyses from Europe, Iran, India, Turkey and Egypt agree that pinocamphone-type oil dominates commerce at roughly 60% to 75% combined ketones, and that linalool, 1,8-cineole, camphor and α-pinene chemotypes exist. The weakness is labeling, not data.

Tier 2, toxicology

Animal convulsant thresholds (Millet, 1979 and 1981), receptor mechanism and metabolism (Höld, 2002), and three human case reports from 1978 and 1981. Consistent, but old, small and all from injected animals or patients dosed at home.

Tier 3, one small human trial

Daneshfard and colleagues (2024): 70 enrolled, 60 completers, 4 weeks, syrup from 12 g of herb a day, with the positive result confined to a post hoc subgroup of 16 and harm in another of 14. Plus one 10-person cosmetic skin study of a cell-culture extract (2026).

Tier 4, rodent and cell studies

Glucose blunting in mice, anticonvulsant extracts in rats, antispasmodic, antifungal and anti-inflammatory activity in tissue and cells, at concentrations not shown to occur in people.

Research Limitations

No measurement of pinocamphone or isopinocamphone in brewed hyssop tea was located, which is the single number that would settle the tea-versus-oil question. No human pharmacokinetic data exist for either ketone. No reproductive study was located. The asthma trial did not report a pre-specified whole-group comparison. The human seizure cases predate modern labeling, and none gives the oil's measured composition. Chemotype is rarely stated in either the studies or the products.

Summary & Key Takeaways

Bottom Line

Hyssop is two products sharing one name. The oil is usually 60% to 75% convulsant ketones and has caused seizures in adults at 10 to 30 drops and in a child given repeated small doses, and no safe oral amount is known; Tisserand and Young's oral ceiling for those ketones is a fraction of one drop. The tea delivers a small, unmeasured share of the same compounds alongside polyphenols that act differently, and its benefits rest on one 4-week Iranian trial in which hyssop helped asthmatics with a productive cough and harmed those with a dry one, in subgroups defined after the fact.

Key Safety Points

• Never swallow hyssop essential oil, and keep it away from children.

• Avoid all hyssop with epilepsy or any seizure history, in pregnancy and while breastfeeding.

• Do not combine the oil with bupropion, tramadol, theophylline, anti-seizure drugs or other ketone-rich oils.

• Asthmatics should not replace or reduce inhalers, and should stop hyssop if cough or breathing worsens.

• For aromatherapy, choose an oil labeled var. decumbens or linalool type with a GC report, and dilute pinocamphone-type oil to 0.3% or less on skin.

Special Note

A seller who prints "Hyssopus officinalis" on an oil without a chemotype or GC report is withholding the one fact that determines its hazard. A tea brand claiming respiratory benefits should be able to say how much pinocamphone a cup contains; on the evidence located for this issue, nobody has published that figure.

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Researched and drafted with AI assistance. Every claim verified against primary sources and human-reviewed before publication. Last reviewed: November 7, 2026. Corrections: reply to any issue.

This issue is educational and is not medical advice. If you take any prescription medicine, especially an anti-seizure drug, bupropion, tramadol or theophylline, have epilepsy, asthma or diabetes, or are pregnant or breastfeeding, talk to a clinician before using hyssop in any form or changing anything.