St John’s Wort is a herbal product taken for low mood. Its own side effects are generally mild. The reason it needs care is different from most supplements: it interacts with more prescription medicines, and more seriously, than almost anything else sold without a prescription.
It speeds up the liver enzymes that break down many medicines, which means those medicines are cleared faster and stop working properly. Because the effect is on the other medicine rather than on you directly, there is often no warning symptom until the treatment fails.
Do not start or stop St John’s Wort without telling whoever prescribes your other medicines. Stopping matters as much as starting, because levels of the other medicine then rise again.
Common side effects reported with this supplement or ingredient
- Stomach upset or nausea
- Tiredness, dizziness or restlessness
- Dry mouth
- Headache
- Increased sensitivity to sunlight, with easier burning, particularly in fair-skinned people or at higher amounts
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.
- Hormonal contraception. Effectiveness can be reduced, with a risk of unintended pregnancy. This includes the pill, and it is one of the most commonly missed interactions. See St John’s Wort and birth control.
- Antidepressants. Combining them can cause serotonin syndrome, which is a medical emergency. See St John’s Wort and antidepressants and serotonin syndrome symptoms.
- Warfarin. Reduced effect, meaning less protection against clots. See warfarin side effects.
- HIV medicines. Reduced levels can allow the virus to rebound and resistance to develop. This is a serious interaction.
- Transplant anti-rejection medicines such as ciclosporin and tacrolimus, where reduced levels risk organ rejection.
- Some epilepsy medicines, some cancer treatments, digoxin and some statins.
See the full St John’s Wort interactions guide. 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 tends to occur.
Low mood that is persistent deserves assessment. See mental health medication side effects if you are weighing up options.
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 vary between products, and herbal products in particular can differ substantially in how much active compound they contain. See how supplements are regulated in South Africa and how to check supplement labels.
Stacking is a common problem: several products each containing the same ingredient add up to more than intended. See taking too many supplements.
Herbal preparations vary considerably in how much active compound they contain, so two products at the same stated dose can behave differently. St John’s Wort also appears within combination mood and sleep formulas, sometimes without being prominent on the front of the pack.
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.
Contact a doctor or pharmacist promptly if you have been taking it alongside any prescription medicine without their knowledge, if a condition being treated seems to be worsening, or if you develop severe sunburn from ordinary sun exposure.
Related supplement safety guides
- Supplement side effects
- Supplement and medicine interactions
- Herbal supplement side effects
- When to see a doctor about a supplement
- Report a supplement side effect in South Africa
See also supplements with antidepressants.
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.