The headline interaction: St. John's wort
The clearest, most important interaction is St. John's wort with hormonal birth control. NCCIH warns that St. John's wort 'can weaken the effects of many medicines,' and birth control pills are specifically listed among them [1]. It does this by speeding up the enzymes that break down the contraceptive hormones, lowering their levels and reducing effectiveness — which can lead to breakthrough bleeding and unintended pregnancy [1].
Why this catches people off guard
St. John's wort is sold over the counter and marketed for mood, so people may not think of it as a 'drug.' But its interaction profile is broad and well documented (see St. John's wort drug interactions), and contraception is one of the highest-stakes examples because the consequence is unintended pregnancy.
Other considerations
- Other enzyme-inducing supplements are less established than St. John's wort, but the general principle — that some supplements lower drug levels — applies (see [herb-drug interactions](/learn/herb-drug-interactions-overview)) [2].
- Birth control can affect nutrient status in the other direction; some research links hormonal contraceptives to modest changes in certain nutrients, a separate topic from effectiveness.
Practical guidance
- If you rely on hormonal birth control, avoid St. John's wort unless your provider confirms it's appropriate and advises a backup method.
- Use a backup contraceptive method if you do take it, per medical guidance.
- Tell your provider and pharmacist about every supplement — especially mood and 'herbal' products — when discussing contraception (see [when to talk to a doctor](/learn/when-to-talk-to-a-doctor-about-supplements)).