A plant with a drug-like interaction profile
Despite being sold over the counter for low mood, St. John's wort has an interaction profile rivaling many prescription drugs. NCCIH states plainly that 'interactions with St. John's wort can weaken the effects of life-saving medicines or cause dangerous side effects' [1].
How it weakens medicines
St. John's wort induces (speeds up) drug-metabolizing enzymes, so the body clears affected medicines faster, lowering their blood levels and effectiveness [1]. NCCIH lists examples it can weaken [1]:
- Birth control pills (risk of unintended pregnancy) — see [supplements and birth control](/learn/supplements-and-birth-control)
- Heart medicines such as digoxin and ivabradine
- HIV medicines such as indinavir and nevirapine
- Cancer medicines such as irinotecan, imatinib, and docetaxel
- Transplant anti-rejection medicines such as cyclosporine — see [supplements and immunosuppressants](/learn/supplements-and-immunosuppressants)
The serotonin risk
Separately, St. John's wort raises serotonin activity, so combining it 'with certain antidepressants or other drugs that affect serotonin...may lead to increased serotonin-related side effects, which can be serious' [1] — see serotonin syndrome and supplements.
Why it's so often missed
Because it's a herbal product for mood, people don't list it as a medication — yet weakening a transplant or HIV drug can have severe consequences. The stakes make disclosure essential [2].
Practical guidance
- Tell every provider and pharmacist if you take or are considering St. John's wort.
- Don't combine it with prescription medicines without medical clearance, especially the categories above.
- Don't start or stop it abruptly around a medication change — stopping it can raise the level of a drug it was speeding up.
- Treat it with the same seriousness as a prescription drug when reviewing interactions.