Drug and Supplement Interactions

Supplement and medicine pairings pharmacists commonly flag

"Natural" does not mean "interaction-free". These are widely documented patterns; always check your own medicines with a pharmacist, because the right advice depends on the exact products and doses.

• Iron, calcium, magnesium and antacids with levothyroxine. Minerals can bind to thyroid medicine in the gut and reduce how much is absorbed. The usual approach is to keep them several hours apart. See our full guide: levothyroxine and iron.
• St John's wort with many medicines. It speeds up how the body clears a range of drugs, which can make the contraceptive pill, warfarin and some other medicines less effective. It can also add to the effects of antidepressants such as SSRIs. Always tell your prescriber if you take it.
• Vitamin K and warfarin. Vitamin K works against warfarin. The aim is a steady intake, not necessarily zero, so tell your anticoagulant clinic before starting or stopping a supplement that contains it.
• Potassium supplements or salt substitutes with ACE inhibitors. Both can raise blood potassium, so this pairing needs medical guidance and sometimes blood tests.
• Minerals with some antibiotics. Iron, calcium, magnesium and zinc can bind to certain antibiotics, including tetracyclines and quinolones, and reduce absorption. Your pharmacist can advise on timing.
• Fish oil, ginkgo and high-dose vitamin E with blood thinners. These may add to bleeding risk when taken alongside anticoagulants or antiplatelet medicines such as aspirin.

A simple routine that avoids most problems

• Keep one up-to-date list of every medicine and supplement you take.
• Show it to a pharmacist whenever you start, stop or change anything.
• Take medicines and supplements at the times your label or pharmacist advises.