What this means in practice
If you are on a combined pill and you start St John's Wort, add a barrier method, and treat bleeding between periods as a signal rather than a nuisance. Nothing here has been tested on progestogen-only methods, so the same caution applies to them by default rather than by evidence.
What people say
A natural mood herb you can buy anywhere. Nothing in the marketing connects it to birth control, and nothing on the shelf tells you to use a condom as well.
What the evidence supports
Four comparative studies exist and three of them found more breakthrough bleeding on the combination; three found significantly lower contraceptive hormone exposure (Berry-Bibee 2016). In 16 women, hormone exposure fell 13% to 15% and follicles grew in a pattern indicating probable ovulation (Murphy 2005). In 17 women on a desogestrel pill, exposure to the progestogen fell by more than 40% and bleeding between periods rose from 35% of women to 77% and 88%, though no one ovulated (Pfrunder 2003). One published pregnancy is attributed to the combination, with four more reported to the German drug authority (Schwarz 2003). The US label for a widely used combined pill names St John's Wort among products that can make it less effective (Lo Loestrin Fe, SPL version 25).
How it is supposed to work
Hyperforin in the extract switches on the liver enzyme CYP3A4 and the intestinal pump P-glycoprotein, both of which clear the oestrogen and progestogen in a combined pill. More clearance means less hormone reaching the blood, which can allow a follicle to develop and can cause bleeding between periods.
Which way it runs
One-directional. St John's Wort lowers the contraceptive's hormone levels; the pill does nothing to St John's Wort. All the human work is on combined pills that contain an oestrogen plus a progestogen. Progestogen-only pills, implants, injections and hormonal coils have not been studied with it, so their status is unknown rather than safe.
What is still missing
The popular version has no opinion about contraception at all, and that silence is the whole problem: nobody is claiming the herb is safe with the pill, so nobody checks, and the two studies that looked disagree about whether ovulation actually breaks through while agreeing that hormone levels drop and bleeding starts.
Sources
- Co-administration of St. John's wort and hormonal contraceptives: a systematic reviewBerry-Bibee EN, Kim MJ, Tepper NK, Riley HE, Curtis KM, 2016
- Interaction of St. John's Wort with oral contraceptives: effects on the pharmacokinetics of norethindrone and ethinyl estradiol, ovarian activity and breakthrough bleedingMurphy PA, Kern SE, Stanczyk FZ, Westhoff CL, 2005
- Interaction of St John's wort with low-dose oral contraceptive therapy: a randomized controlled trialPfrunder A, Schiesser M, Gerber S, Haschke M, Bitzer J, Drewe J, 2003
- LO LOESTRIN FE (norethindrone acetate and ethinyl estradiol, ethinyl estradiol and ferrous fumarate): US FDA prescribing informationAllergan, an AbbVie company, SPL version 25
- St John's wort (Hypericum perforatum): drug interactions and clinical outcomesHenderson L, Yue QY, Bergquist C, Gerden B, Arlett P, 2002
- Herb-drug interactions with St John's wort (Hypericum perforatum): an update on clinical observationsBorrelli F, Izzo AA, 2009