Why is hyssop essential oil linked to seizures?

The evidence doesn't support hyssop essential oil as safe to take internally: a 2019 systematic review named hyssop, among other essential oils, as able to trigger seizures when taken by mouth, due to convulsant compounds it contains. That documented risk is specific to the concentrated oil — not to hyssop tea or the dried herb at ordinary food-use amounts.

What the evidence doesn't support

It doesn't support taking hyssop essential oil internally, at any amount — this page gives no guidance for that use. It doesn't support treating hyssop tea, capsules or a typical liquid extract the same way as the oil; the seizure risk documented in the research below is tied to the oil's concentration, not to hyssop as a plant generally. And it doesn't support hyssop, in any form, as a treatment for seizures or epilepsy — no research has looked at that question, and none of the sources here suggest it.

What the trials found

A 2019 systematic review examined internal use of essential oils and seizure risk, drawing on human case reports, animal studies and in vitro research. It named hyssop, alongside sage, rosemary, camphor, pennyroyal, eucalyptus, cedar, thuja and fennel oils, as able to cause seizures when taken internally, because these oils contain convulsant compounds — in hyssop's case, mainly pinocamphone and related ketones. The review's proposed mechanism is that these compounds interfere with GABA-related signaling in the nervous system and affect ion channel activity, though it's a review of existing reports and lab work rather than a controlled trial that measured seizure rates in a defined group of people. It doesn't give a specific oral amount of hyssop oil that is safe, which is part of why this page doesn't offer one.

Separately, the one published human trial of hyssop — a 2024 randomized, placebo-controlled trial of a concentrated extract syrup in 60 adults with diagnosed asthma — excluded anyone with a seizure history from participating, as a precaution. That trial tested a syrup, not the essential oil, and reported no seizures in either group; its exclusion criterion reflects the same caution documented in the 2019 review, not a new finding of its own.

Questions people ask

Why does hyssop cause seizures?

The concern is specific to the essential oil, not hyssop as a plant. Steam distillation concentrates pinocamphone and related ketone compounds from the leaves and flowers into a small volume of oil — a much higher concentration than a cup of tea or a capsule of dried herb delivers. The 2019 systematic review's working explanation is that these compounds disrupt GABA-related nerve signaling, which is one known way a substance can trigger a seizure.

Is hyssop tea safe if I'm worried about seizures?

Hyssop tea is made from the dried leaf and flowering tops at food-use amounts — a different, far less concentrated product than the essential oil. FDA lists hyssop among spices generally recognized as safe for food use (21 CFR 182.10). The documented seizure risk is specific to the concentrated oil taken internally, not to tea. Anyone with a seizure history who wants extra reassurance should still talk to a doctor before using hyssop in any form, since no dedicated safety trial has tested tea in that population either.

How much hyssop essential oil causes a seizure?

There's no established safe internal amount. The 2019 review didn't set a specific safe or unsafe human dose, which is why this page — and hyssop essential oil on skin — gives no guidance for taking the oil by mouth at all.

Is hyssop oil different from hyssop tea for this risk?

Yes. Hyssop essential oil is steam-distilled and concentrates the compounds in question; hyssop tea is a hot-water infusion of the dried herb at ordinary culinary amounts. See hyssop tea and hyssop essential oil on skin for how the two products differ.

When to talk to a doctor

Anyone with a history of seizures or epilepsy should talk to a doctor before using hyssop in any form, and should not take hyssop essential oil internally. Hyssop is not a treatment for seizures or epilepsy, and nothing in the research summarized here suggests it is; a supplement is not a substitute for anti-seizure medication or medical care. Anyone who takes anti-seizure medication or has any other condition affecting the nervous system should raise hyssop, in any form, with a doctor or pharmacist before using it.

Related reading: hyssop side effects, who should not take hyssop, and hyssop essential oil on skin.