Interaction fact sheet
St John's wort and the contraceptive pill
Combination to avoid: the pill may lose its effectiveness.
St John's wort speeds up clearance of the hormones in the pill. In two trials, it caused bleeding between periods and, in one of them, signs of ovulation. If you take St John's wort, the pill alone is no longer sufficient contraception.
In brief
- Mechanism
- St John's wort activates enzymes in the liver and intestine, especially CYP3A4, which clear the pill's hormones faster.
- Magnitude
- A 13–15% reduction in hormone exposure; ethinylestradiol half-life halved in one trial.
- Who is affected
- Anyone using hormonal contraception: the pill, but also implants, patches or rings.
- What to do
- Avoid St John's wort, or use non-hormonal contraception while taking it and for about two weeks after stopping.
What the trials show
In a trial published in 2003, 12 women taking the combined pill took 300 mg of St John's wort three times a day for two cycles. The half-life of ethinylestradiol fell from around 23 hours to 12 hours, and bleeding between periods increased.
A second trial, involving 16 women on a low-dose pill, measured a 13–15% reduction in hormone exposure. It also observed follicle growth and signs of probable ovulation during cycles with St John's wort. That is exactly what the pill is supposed to prevent.
Why this happens
St John's wort, particularly its compound called hyperforin, increases the activity of CYP3A4 and P-glycoprotein, two systems that clear many medicines from the body. The hormones in the pill are therefore broken down faster and their concentrations fall. The effect develops over about ten days and persists for around two weeks after stopping.
The same mechanism affects other medicines: anticoagulants, immunosuppressants, certain HIV treatments and many others. It is one of the herbs with the most drug interactions.
In practice
What you can do
- If you use hormonal contraception, avoid St John's wort.
- If you're already taking it, use a condom as well while taking it and for about two weeks after stopping.
- Always tell your doctor or pharmacist about St John's wort, just as you would a medicine.
When to seek advice
- Unusual bleeding between periods while on the pill.
- A late period after taking St John's wort.
- Depressed mood: treatments are available that do not interact with contraception.
Main sources
- 01Hall SD et al. The interaction between St John's wort and an oral contraceptive. Clinical Pharmacology & Therapeutics, 2003. PubMed
- 02Murphy PA et al. Interaction of St. John's Wort with oral contraceptives: effects on the pharmacokinetics of norethindrone and ethinyl estradiol, ovarian activity and breakthrough bleeding. Contraception, 2005. PubMed
Frequently Asked Questions (FAQ)
Does St John's wort cancel out the pill?
It lowers the pill's hormone concentration and caused signs of ovulation in one trial. It does not necessarily make it ineffective in every woman, but pregnancy risk increases. You can no longer rely on the pill alone.
How long after stopping St John's wort does the pill become reliable again?
The effect on enzymes persists for around two weeks after the last dose. Keep using additional contraception during this period.
Are implants and hormonal IUDs affected?
Implants, patches and rings are affected because they rely on the same hormones. Hormonal IUDs act mainly locally, but ask your doctor for advice. Copper IUDs are not affected.