Hyssop side effects

The dried herb and the essential oil have very different side-effect pictures. In the one human trial of an oral hyssop extract, side effects were mild — thirst, dry mouth and throat irritation. Hyssop essential oil is a separate matter: taken internally, it has been linked to seizures, because it concentrates a compound called pinocamphone.

What the one human trial reported

The 2024 randomized trial that gave adults with asthma a hyssop extract syrup (12 g/day) for 4 weeks recorded thirst, dry mouth and throat irritation as side effects in the hyssop group. Five participants — all of whom had a dry cough rather than a productive one — discontinued the syrup because their coughing and shortness of breath got worse. That's the only side-effect data that comes from an actual human trial of hyssop taken by mouth; nothing else has been systematically recorded in people.

Essential oil: a different risk entirely

Hyssop essential oil is distilled to concentrate pinocamphone and isopinocamphone. A 2019 systematic review of essential oils and seizure risk named hyssop among the oils — alongside sage, rosemary, camphor and pennyroyal — that can trigger seizures when taken internally, because these compounds act as convulsive agents. This is specific to the concentrated oil, not the dried herb or tea; the concentrations involved are entirely different. If you have a history of seizures, talk to a doctor before using hyssop essential oil in any way, and don't take it internally.

Liver

The National Institutes of Health's LiverTox reference lists hyssop as widely used and not implicated in causing liver injury. That's a reassuring absence of a known problem, not the same as a study that tested for one directly — no dedicated human safety trial has looked at hyssop and liver function.

Allergy and cross-reactivity

Hyssop is in the mint family, alongside sage, peppermint, thyme, lavender and savory. The one human trial excluded anyone with a known allergy to any of those herbs as a precaution, which is a reasonable practical rule of thumb if you react to other mint-family herbs, even though it wasn't a finding about hyssop allergy specifically.

Questions people ask

Does hyssop tea have side effects?

No trial has tested hyssop tea specifically. The closest data point is the one syrup-extract trial, where the side effects reported were thirst, dry mouth and throat irritation.

Can hyssop cause seizures?

The essential oil, taken internally, has been documented as a seizure risk because of its pinocamphone content — a 2019 review grouped it with several other essential oils that share this risk. That risk is specific to the concentrated oil; it hasn't been reported for the dried herb, tea or the one syrup extract that's been tested in a human trial.

Related reading: who should not take hyssop, hyssop and medications: what to check, and how much hyssop should you take a day?.