What's happening
Hyperforin, a primary constituent of St John's wort, activates the pregnane X receptor (PXR) and induces cytochrome P450 3A4 and P-glycoprotein. CYP3A4 metabolizes ethinylestradiol and most progestins; inducing it accelerates contraceptive clearance and lowers circulating hormone levels. A direct RCT (Pfrunder 2003, N=17 healthy women on a low-dose combined OC) found breakthrough bleeding rose from 6/17 (35%) on OC alone to 13/17 (77%, p<0.015) with SJW 300 mg twice daily and 15/17 (88%, p<0.001) with SJW 300 mg three times daily — a clean dose-response pattern. Ovulation was not detected but follicle-like structures appeared sporadically, meaning the contraceptive margin narrows. Pharmacovigilance confirms the clinical impact: by December 2001 the UK and Sweden together had received 9 reports of unintended pregnancies attributed to SJW + OC co-use, all in women who had used OCs without problems for at least seven months before adding SJW. Products with greater than 1 mg hyperforin/day are considered high-risk; a low-hyperforin extract study (Will-Shahab 2009) showed no measurable PK interaction, but most retail SJW products are high-hyperforin and hyperforin content is rarely disclosed on US labels. CYP3A4 induction persists for 1-2 weeks after stopping SJW. Progestin-only pills, patches, rings, implants, hormonal IUDs, and levonorgestrel emergency contraception are all metabolized by CYP3A4 and likely affected; direct trial data on non-combined methods are sparse.
Recommendation
If you rely on oral contraception, do not take St John's wort without discussing it with your prescriber. If you are taking both, use a reliable backup method (condom, non-hormonal IUD, abstinence) during SJW use and for at least two weeks after stopping. Breakthrough bleeding on a previously stable OC regimen is a warning sign. For levonorgestrel emergency contraception, SJW use in the prior 1-2 weeks reduces efficacy — a copper IUD is a more reliable alternative.
Timing
Spacing doses does not help. CYP3A4 induction is a transcriptional effect developing over days and resolving over days — unrelated to hour-by-hour timing.
Sources
- — PMID:12911359 — Pfrunder A et al. Interaction of St John's wort with low-dose oral contraceptive therapy: a randomized controlled trial. Br J Clin Pharmacol 2003.
- — PMID:17486092 — Borrelli F, Izzo AA. St John's wort (Hypericum perforatum): drug interactions and clinical outcomes. CNS Drugs 2007.
- — PMID:31943241 — Nicolussi S et al. Clinical relevance of St John's wort drug interactions revisited. Br J Pharmacol 2020.
- — https://pmc.ncbi.nlm.nih.gov/articles/PMC11283811/ — Co-administration of St John's wort and hormonal contraceptives: a systematic review. 2024.