Herbal medicines and prescription interactions: how to check before you start

A herbal product contains many chemicals, some of which affect liver enzymes, blood clotting, blood pressure, the nervous system or blood sugar. When these effects meet those of a prescription medicine, the result can be a weaker or stronger effect than intended. Because the content of herbal products varies, the size of the effect can be hard to predict.

Unlike prescription medicines, many herbal products have not been tested for interactions in the same way. NHS pages on several medicines state that complementary medicines, herbal remedies and supplements are not tested in the same way and that there is not enough information to say they are safe together.

St John's wort is sold for low mood and appears on the NHS interaction pages for warfarin, apixaban, rivaroxaban, atorvastatin, amlodipine, levothyroxine, omeprazole, sertraline, citalopram and paroxetine. The pattern is that it can reduce the effect of some medicines and increase serotonin effects with antidepressants.

Our guide on enzyme-inducing medicines explains the mechanism, and the condition guide on St John's wort for depression describes the evidence. If you want to stop St John's wort, tell the prescriber, because medicine doses may need adjusting afterwards.

The NHS herbal medicines page says to check first with a pharmacist before taking a herbal medicine if you already take other medicine, to consult your medicine packet leaflet or the electronic medicines compendium and to tell all healthcare providers about herbal use. It says herbal medicines are not recommended for children, and are unsafe for pregnant or breastfeeding individuals and for those with kidney or liver problems.

The NHS lists prescription medicines affected including statins, antidepressants, diabetes medicines such as metformin, blood pressure tablets such as ramipril, blood thinners such as warfarin and hormonal contraceptives.

• Ask a pharmacist before starting.

• Read the leaflet of your prescription medicine for herbal warnings.

• Tell your GP, dentist and surgeon about herbal products.

• Avoid herbal products in pregnancy unless advised.

The NHS says to look for a traditional herbal registration (THR) mark and a product code from the MHRA. The registration indicates the product has met standards of quality and safety information and carries a leaflet with usage and warnings. It does not mean that the herb is proven to treat your condition or free of interactions.

Products with no THR mark may be unlicensed or sold as food supplements, with different oversight. The NHS says not to buy unlicensed herbal medicines online as they may pose serious risks.

Medicines with a narrow margin between benefit and harm deserve extra care: warfarin and other anticoagulants, lithium, antiepileptics, immunosuppressants after transplant, hormonal contraception and cancer treatments. The NHS lithium page says there is not enough information to say herbal products are safe with lithium. Cancer Research UK says cancer drugs can interact with medicines, herbal products and some food and drinks, and advises telling the healthcare team about herbal supplements.

If you are on any of these, ask before taking any herbal or supplement product, even a tea sold as a wellbeing blend.

Write down the product name and its ingredients, not just the brand. Ask the pharmacist to check against all your prescriptions and any over-the-counter medicines. Keep the box and leaflet so you can show them. Start one product at a time so you can link any change to its cause, and stop and call a professional if new symptoms occur.

When you attend any appointment, include herbal products on the medicines list you give. Many interactions are only found because someone mentioned them.