St. John's Wort

📜 Historical Use

St John's wort (Hypericum perforatum) takes its name from its flowering around the feast of St John the Baptist in late June. It has a long history of use for low mood in European folk medicine, and it remains one of the most-studied botanicals in psychiatry — which is why its interaction profile is documented in far more detail than that of most herbal products.

📊 What the Research Shows

For mild or moderate depression, St John's wort appears more effective than placebo and as effective as standard antidepressant medications — which is a genuinely notable result for a botanical. What is not established is whether it helps severe depression, or whether the benefit holds beyond 12 weeks. There is limited evidence for menopausal hot flashes and for somatic symptom disorder. Research on ADHD, irritable bowel syndrome, obsessive-compulsive disorder and premenstrual syndrome has not clearly shown effectiveness. The catch is that the population most likely to reach for it — people already taking prescription medication — is the population for whom it is least appropriate.

It is sold as a standardized extract and taken daily for several weeks before any effect can be judged. Trials generally ran to about 12 weeks, and it is not established whether the benefit holds beyond that. Before you start, go through the interactions section below with a pharmacist against every medicine you take, including contraception — on this herb that check matters more than the dose does, and it needs to happen before rather than after.

Interactions are the main safety story here, and they are the reason to read the page: St John's wort can weaken the effect of life-saving medicines. It can reduce the effectiveness of birth control pills, so unplanned pregnancy is a real risk. Taken alongside antidepressants or other drugs affecting serotonin it can cause serious serotonin-related effects, including serotonin syndrome. It can also cause severe photosensitivity — an exaggerated burn response to sunlight, not mild sensitivity — and may be unsafe in pregnancy because it may increase the risk of birth defects.

⚠️ What happens if taken in excess:

For adults taking no other medications, St John's wort appears safe for up to 12 weeks, and some studies indicate it can be used safely for a year or more. Possible side effects include severe photosensitivity reactions — meaning a genuinely exaggerated burn response to sunlight, not mild sensitivity — along with diarrhoea, dizziness, insomnia, restlessness and skin tingling. It may be unsafe during pregnancy because it may increase the risk of birth defects. Nursing infants of mothers taking it can experience colic, drowsiness and lethargy.

This is the section that matters, and it is unusually long for good reason: interactions with St John's wort can weaken the effects of life-saving medicines or cause dangerous side effects. Documented interactions include antidepressants (amitriptyline, bupropion), birth control pills, cyclosporine used to prevent organ transplant rejection, anticonvulsants (phenytoin, carbamazepine), cardiac drugs (digoxin, ivabradine), HIV medicines (indinavir, nevirapine), cancer drugs (irinotecan, imatinib, docetaxel), the anticoagulant warfarin, and statins such as simvastatin. Two consequences deserve spelling out. Because it can reduce the effectiveness of birth control pills, unplanned pregnancy is a real risk. And taken alongside certain antidepressants or other drugs affecting serotonin, it may lead to increased serotonin-related side effects which can be serious — the syndrome to be aware of is serotonin syndrome. If you take any prescription medicine at all, this is a herb to discuss with a pharmacist before, not after.

1. St. John's Wort — What the Science Says — National Center for Complementary and Integrative Health NIH NCCIH (2026)[NIH ↗]