St. John's Wort Interactions With Prescription Drugs

St. John's wort is one of the most widely used herbal supplements in the world, but it also has one of the longest lists of clinically significant drug interactions of any botanical product. If you take any prescription medication, reading this guide before considering St. John's wort may help you avoid a serious and potentially dangerous outcome.

What Is St. John's Wort?

St. John's wort (Hypericum perforatum) is a flowering plant that has been used traditionally for low mood, wound healing, and other complaints. It is not an essential nutrient, and according to the NIH Office of Dietary Supplements, no adult target intake or upper limit has been established because it has no established role in normal human physiology.

Laboratory work suggests that constituents of the plant may affect chemical messengers such as serotonin in the brain, though the exact mechanism in people is not fully established. As with all dietary supplements, St. John's wort products are not intended to diagnose, treat, cure, or prevent any disease, and these statements have not been evaluated by the FDA.

Is St. John's Wort Safe on Its Own?

For most adults who are not taking any medicines, St. John's wort appears to be safe when used by mouth for up to 12 weeks, according to NIH data. The safety of longer use is not established. The picture changes substantially, however, the moment other medications enter the picture. The NIH notes that St. John's wort interacts with a very large number of medicines and can weaken or dangerously alter their effects. It may also be unsafe during pregnancy. Always speak with a clinician or pharmacist before starting St. John's wort, especially if you are pregnant, nursing, or managing a chronic condition.

How St. John's Wort Interferes With Medications

St. John's wort is a potent inducer of certain liver enzymes and transport proteins that the body uses to process and eliminate drugs. When these pathways are activated more strongly than usual, many medications are broken down faster than intended, which lowers their concentration in the blood and reduces their effectiveness. In other situations, combining St. John's wort with drugs that also affect serotonin can push serotonin activity to dangerous levels rather than reduce it.

Key Drug Interactions to Know

Antidepressants

This is among the most serious interaction categories. Combining St. John's wort with antidepressants, including SSRIs, SNRIs, tricyclic antidepressants such as amitriptyline, and bupropion, can raise serotonin to dangerous levels and cause serotonin syndrome. Serotonin syndrome is a potentially life-threatening condition that can involve agitation, rapid heart rate, high blood pressure, muscle rigidity, and confusion. If you are currently taking any antidepressant, do not add St. John's wort without direct guidance from your prescribing doctor.

Birth Control Pills

St. John's wort can reduce the effectiveness of oral contraceptives by speeding up the liver enzymes that break them down. This can lead to breakthrough bleeding and, critically, unintended pregnancy. People relying on hormonal contraception for pregnancy prevention should discuss this interaction with their doctor or pharmacist before using St. John's wort in any form.

Transplant Anti-Rejection Drugs

Cyclosporine and similar drugs used to prevent organ rejection after a transplant are particularly vulnerable to this interaction. St. John's wort can cause blood levels of cyclosporine to drop significantly, which raises the risk of the body rejecting the transplanted organ. This is considered a high-stakes interaction and one of the clearest reasons transplant recipients should avoid St. John's wort entirely unless explicitly directed otherwise by their transplant team.

Warfarin and Other Anticoagulants

Warfarin requires careful dosing to keep blood-clotting levels within a narrow therapeutic range. St. John's wort can reduce the blood-thinning effect of warfarin, potentially allowing dangerous clots to form. Anyone on anticoagulant therapy needs to inform their prescriber and pharmacist before making any changes to their supplement routine, including starting or stopping St. John's wort.

Other Medications

The interactions listed above are not exhaustive. St. John's wort has been reported to affect the blood levels of HIV medications, certain cancer drugs, sedatives, and other compounds that rely on the same liver enzymes. If you take any prescription drug regularly, ask your pharmacist specifically whether St. John's wort is a concern for your medication regimen.

What Does the Evidence Say About Effectiveness?

Understanding the interaction risk is important, but so is having an honest picture of what St. John's wort may or may not do. The NIH grades the evidence as follows:

• Mild to moderate depression (Moderate evidence): Research suggests St. John's wort may be more effective than placebo and roughly as effective as standard antidepressants for mild to moderate depression. This is the best-supported use in the current literature.
• Severe depression or long-term use (Insufficient evidence): Evidence is insufficient to support using St. John's wort for severe depression or for periods extending beyond approximately 12 weeks.
• Menopausal symptoms and hot flashes (Limited evidence): Some limited evidence exists, but findings are not strong enough to draw firm conclusions.
• ADHD, irritable bowel syndrome, OCD, premenstrual syndrome, somatic symptom disorder, and smoking cessation (Insufficient evidence): Current evidence is insufficient to support St. John's wort for any of these conditions.
• Topical use for wound healing or skin conditions (Limited evidence): Limited evidence suggests possible benefit when applied to the skin, but more research is needed.

Evidence ratings above reflect NIH assessments sourced from the National Center for Complementary and Integrative Health and the NIH Office of Dietary Supplements.

Practical Steps Before You Consider St. John's Wort

• Make a complete list of every prescription drug, over-the-counter medicine, and supplement you currently take.
• Bring that list to your doctor or pharmacist and ask specifically about St. John's wort interactions before starting.
• Do not stop or adjust any prescription medication on your own in order to try St. John's wort.
• If you are pregnant, planning to become pregnant, or breastfeeding, discuss any supplement use with your healthcare provider first.
• Remember that no established dosing target exists for St. John's wort, so personal guidance from a clinician or pharmacist is especially important.

The Bottom Line

St. John's wort has a meaningful body of research behind its use for mild to moderate depression, but it also carries a substantial and well-documented risk of interacting with prescription medications. The NIH is clear that it should not be combined with prescription drugs without medical advice. If you are considering it, a conversation with your doctor or pharmacist is not optional, it is essential.