St John’s Wort interacts with more prescription medicines, and more seriously, than any other supplement commonly sold without a prescription. Its own side effects are mild. The interactions are the reason it needs care.
How it works matters here. St John’s Wort speeds up liver enzymes that break down many medicines, so those medicines are cleared faster and their levels fall. Because the effect is on the other medicine rather than on you directly, there is usually no warning symptom. The first sign is often the other treatment failing.
Stopping matters as much as starting. When you stop, levels of the other medicine rise again, which can push them too high. Neither should be done without telling whoever prescribes your medicines.
Common side effects reported with this supplement or ingredient
Stomach upset, tiredness, restlessness, dry mouth, headache, and increased sensitivity to sunlight. See St John’s Wort side effects.
Possible interactions with medicines, chronic conditions or pregnancy
This list is not exhaustive. If you take any regular medicine, check before using St John’s Wort at all.
Where reduced effectiveness is the danger:
- Hormonal contraception. Risk of unintended pregnancy. One of the most commonly missed interactions. See St John’s Wort and birth control.
- HIV medicines. Reduced levels can allow the virus to rebound and resistance to develop.
- Transplant anti-rejection medicines such as ciclosporin and tacrolimus, where reduced levels risk organ rejection.
- Warfarin. Reduced effect means less protection against clots. See warfarin side effects.
- Some epilepsy medicines, where reduced levels can mean seizures.
- Some cancer treatments, digoxin, and some statins.
Where added effect is the danger:
- Antidepressants. Combining can cause serotonin syndrome, a medical emergency. See St John’s Wort and antidepressants.
- Triptans for migraine, and tramadol, for the same reason.
Avoid in pregnancy and breastfeeding unless specifically advised.
Who should speak to a healthcare professional before using it?
Anyone taking any regular prescription medicine, anyone using hormonal contraception, and anyone being treated for depression or bipolar disorder. Taking a herbal antidepressant instead of prescribed treatment, or alongside it without telling anyone, is where the real harm occurs.
Also tell your surgeon and anaesthetist before an operation. See supplements before surgery.
Dose, product-quality and stacking risks
Supplements in South Africa are regulated as complementary medicines, and the checks applied are not the same as those for prescription medicines. Strength, purity and labelling can vary between products. See how supplements are regulated in South Africa and how to check supplement labels.
Herbal preparations vary considerably in active content, so the size of an interaction is not predictable from the label. St John’s Wort also appears inside combination mood and sleep formulas, sometimes without being prominent on the front of the pack, so read the full ingredient list.
When to stop and get medical advice
Get emergency help for agitation or confusion with fever, sweating, a racing heart, muscle twitching and diarrhoea, which can indicate serotonin syndrome. See serotonin syndrome symptoms.
Contact a doctor or pharmacist promptly if you have been taking it alongside a prescription medicine without their knowledge, or if a condition being treated appears to be worsening. Do not stop the prescription medicine; stop the St John’s Wort and ask, since levels will shift either way and may need monitoring.
Related supplement safety guides
- Supplement and medicine interactions
- Taking too many supplements
- When to see a doctor about a supplement
- Unsafe supplements in South Africa
- Report a supplement side effect in South Africa
Medical Disclaimer: This content is for informational and educational purposes only and does not constitute medical diagnosis, prescription, or professional advice. Always read the patient information leaflet and consult a qualified doctor or pharmacist regarding any medicine or health concern.