St John's Wort Has Been Linked to Contraceptive Failure

St John’s Wort is one of the most studied herbal supplements in the context of drug interactions. Its interaction with oral contraceptives carries real-world consequences that go beyond pharmacology.

The mechanism

Oral contraceptives — whether combined oestrogen-progestogen pills or progestogen-only pills — are metabolised primarily by the CYP3A4 enzyme system in the liver. When this enzyme system is more active than usual, contraceptive hormones are broken down faster, resulting in lower blood concentrations.

St John’s Wort is a potent inducer of CYP3A4. Clinical pharmacokinetic studies have confirmed that it can significantly reduce the area under the curve (AUC) of synthetic oestrogens and progestogens, meaning substantially less hormone is circulating at any given time.

The clinical evidence

A 2000 paper in the Lancet by Yue, Bergquist, and Gerden reviewed pharmacokinetic data and case reports, including instances of breakthrough bleeding in women taking oral contraceptives who had started St John’s Wort. Breakthrough bleeding is a recognised sign that hormonal levels have dropped below the threshold required for ovulation suppression.

The UK’s Committee on Safety of Medicines issued guidance in 2000 warning that St John’s Wort should not be taken with oral contraceptives, hormone replacement therapy, or a range of other medications, including certain antivirals, anticoagulants, and cyclosporine.

The population at risk

Women who use herbal supplements are more likely, not less likely, to be health-conscious and to think of “natural” products as safe. The irony is that St John’s Wort, specifically because of its enzymatic activity, poses a consistent and documented risk to one of the most commonly used pharmaceutical interventions in reproductive-age women.

The interaction is not trivial. Unlike some drug-supplement interactions where the clinical significance is debated, this one has been confirmed in multiple pharmacokinetic studies showing measurable hormone reduction at doses of St John’s Wort typically sold in health food stores and pharmacies.

What to do

Anyone taking an oral contraceptive as their primary method of pregnancy prevention should not take St John’s Wort. If St John’s Wort is started, a backup method should be used for at least the duration of use and for several weeks after stopping, as CYP3A4 induction takes time to reverse.

This guidance applies to progestogen-only pills and the contraceptive patch as well, both of which use hormones metabolised by the same enzyme pathway.

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Reference

1. Yue QY, Bergquist C, Gerden B. "Safety of St John's Wort (Hypericum perforatum)." Lancet, 2000. PubMed 10722791