Drug interactions
Isoniazid + St john's wort
Severity: Moderate
Isoniazid and St john's wort: moderate interaction.
Last updated
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
St John's wort has serotonergic activity, and isoniazid has weak MAO inhibition, so combined use might increase serotonergic effects. St John's wort also induces CYP enzymes and P-glycoprotein, which may alter the levels of other drugs, although isoniazid is mainly cleared by acetylation and not by these pathways.
What this interaction means in plain words
Isoniazid and St john's wort can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken.
Why it happens
Additive serotonergic effects from St John's wort and weak MAO inhibition by isoniazid.
What to do
Advise patients to avoid St John's wort during isoniazid therapy. Monitor for serotonin toxicity if the herbal product is used.
What to ask your pharmacist
- Should I take them at different times of day?
- Does the dose of either medicine need to change?
- Is there anything I should avoid, such as alcohol or certain foods, while on both?
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Other interactions of Isoniazid
Frequently asked questions
Can I take Isoniazid with St john's wort?
MediHelp rates the interaction as moderate. Isoniazid and St john's wort can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken. St John's wort has serotonergic activity, and isoniazid has weak MAO inhibition, so combined use might increase serotonergic effects. St John's wort also induces CYP enzymes and P-glycoprotein, which may alter the levels of other drugs, although isoniazid is mainly cleared by acetylation and not by these pathways.
How serious is the interaction between Isoniazid and St john's wort?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Advise patients to avoid St John's wort during isoniazid therapy. Monitor for serotonin toxicity if the herbal product is used.
What other medicines interact with Isoniazid?
MediHelp also lists interactions of Isoniazid with, for example, Bacillus calmette-guerin substrain tice, Benzhydrocodone / acetaminophen, Carbamazepine. The full list is on the Isoniazid page.
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