Supplements and medicines: combinations worth knowing about

Supplements affect liver enzymes, absorption and clotting. Below are the combinations that come up most often in guidelines and interaction references.

Five common situations

St John's wort and a long list of drugs. It induces cytochrome P450 enzymes and can lower levels of antidepressants, oral contraceptives, and some antiretroviral and immunosuppressant drugs. It is among the best-documented interactions of any herbal product.

Vitamin K and warfarin. Sudden changes in vitamin K intake shift the INR. The point is not to avoid greens but to keep intake steady and monitor the number.

Calcium, iron, magnesium with antibiotics or levothyroxine. These minerals bind drugs in the gut and reduce absorption. Separating doses by a few hours is the usual advice.

High-dose omega-3, ginkgo, garlic with anticoagulants. Effects on clotting can add up. Surgeons want to know about these before a planned operation.

Potassium and magnesium with ACE inhibitors or potassium-sparing diuretics. Risk of hyperkalaemia, especially with reduced kidney function.

What to do in practice

• Keep one list: prescriptions, over-the-counter products, supplements, herbal teas.
• Show that list, in full, to your doctor and pharmacist.
• Mention supplements before any planned surgery.
• Introduce new products one at a time so you can tell what caused what.

This article is not a substitute for that conversation. It is about which questions to bring to it.