Can supplements interact with medicines or each other?

How an interaction can happen

A supplement can change how a medicine is absorbed, broken down or removed from the body. This can make the medicine more or less active. Other combinations can add to the same effect or side effect. “Natural” does not by itself mean free of interactions. Some medicines can also affect nutrient absorption; see B12 and metformin.

Another example is that prolonged use of some acid-reducing medicines may lower magnesium levels. Read magnesium and proton pump inhibitors.

Read how iron, calcium and biotin can affect thyroid medicine or tests.345910

One example: vitamin K and warfarin

Vitamin K can interact with warfarin, a medicine that helps prevent blood clots. Sudden changes in the amount of vitamin K you get from food or supplements can change how warfarin works. If you take warfarin, discuss changes with the clinician managing your treatment. Do not stop eating vitamin K foods or change your medicine on your own. For a common combination claim, see vitamin D3 and K2.

Warfarin works by opposing vitamin K. NHS guidance says people taking warfarin can still eat vitamin K-rich foods such as greens; consistency and discussing major changes with their anticoagulation team matter. The same food restrictions do not usually apply to direct oral anticoagulants such as apixaban, but other medicine and supplement interactions remain possible. Ask about the specific anticoagulant and product; never change prescribed treatment yourself.34511

Magnesium, antibiotics and osteoporosis medicines

Magnesium can bind to some antibiotics in the gut, so less of the medicine is absorbed. Examples include tetracyclines and quinolones. What to do depends on the exact products. Ask a pharmacist how to take them; do not guess how far apart to take them or stop treatment. See the magnesium profile.

The NIH gives doxycycline and ciprofloxacin as examples of antibiotics affected by magnesium. Magnesium-containing supplements or medicines can also reduce absorption of oral alendronate, used for osteoporosis. Magnesium is found in some antacids and laxatives as well as supplements. The timing rules are not the same for every product; check the exact combination with a pharmacist.

Iron is another mineral to check: UK NHS guidance lists doxycycline and ciprofloxacin among medicines that can interact with ferrous sulfate. Tell the pharmacist or prescriber about the exact iron product, including any multivitamin, and ask how to manage timing. Do not stop an antibiotic or change prescribed treatment on your own. See iron, deficiency and safety.26

St John’s wort can change several medicines

St John’s wort, also called Hypericum perforatum, is an herbal product sometimes used for mood. It can reduce the effectiveness of medicines including some hormonal contraceptive pills, HIV treatments and medicines used after organ transplantation. It can also interact with certain antidepressants in a different way, increasing the risk of serious serotonin-related effects. The exact interaction depends on the medicines and product. Tell a pharmacist or prescriber if you already take it or are considering it; ask for individual advice before starting or stopping it or changing prescribed treatment. For the research on mood and its limits, see St John’s wort and depression.

For example, the US prescribing information for Eliquis (apixaban) says to avoid taking it with St John’s wort: the herb can reduce apixaban exposure and increase the risk of stroke or other blood clots. This is a different interaction from vitamin K and warfarin. If you already take both, contact the prescribing team or pharmacist promptly for a plan; do not stop the anticoagulant on your own. Product labels and advice can vary by country, so check the exact medicine and local guidance. For another serotonin-related supplement, see 5-HTP and its interactions.

If you need emergency contraception, seek advice promptly and tell the clinician or pharmacist if you use St John’s wort or have used it recently. UK MHRA guidance says its effect on some emergency contraceptive pills can remain relevant after stopping, including use within the previous four weeks; an alternative method may be recommended. The right choice depends on timing and circumstances, so do not select a dose or method based on this example alone.831213

Biotin can affect a laboratory result

Biotin is found in some hair and nail supplements. It can affect certain laboratory tests. The US FDA has received reports of falsely low results in some troponin tests, which can help assess heart damage. Not all tests are affected. Tell the healthcare team and laboratory about any supplements you take. Follow their advice, and do not delay urgent care. Read the biotin profile for the evidence and test limitations.7

Two supplements may repeat an ingredient

A multivitamin, a fortified drink and a separate supplement can contain the same nutrient. Compare labels and serving sizes, including the amount actually used, and show the full list to a pharmacist or other health professional. Overlap does not prove harm, but it can make the total higher than expected. For a specific example, see vitamin B6 in multiple products. For an example involving cod liver oil, multivitamins and some medicines, see vitamin A and overlapping products. For a form-specific example involving orlistat and oral retinoids, see vitamin A interactions.345

Prepare for a useful conversation

List prescription and non-prescription medicines, supplements and herbal products, with the product names, ingredients, strengths and how you use them. Include any planned changes and relevant upcoming surgery or tests. Ask about specific interactions before adding or stopping a product; do not replace prescribed treatment with a supplement.

Before surgery, tell the surgeon and anaesthesia team about all supplements and herbal products as far in advance as possible. Some can affect anaesthetic or other medicines, increase bleeding, or change heart rate or blood pressure. Ask the team whether anything needs to be stopped; do not stop prescribed medicines without advice. In an emergency, tell the team or ask someone with you to do so.1345

What this guide cannot check

Interactions vary by product and person, and not all are known. A pharmacist or prescriber can check your combination and advise you if you think you are having a side effect. Seek medical advice promptly for a concerning reaction.345