Can supplements interact with prescription medication?

Science writer James Witts explains how some supplements can interfere with prescription medicines and why it's always important to seek medical advice.

can supplements affect prescription medicine

The amount of over-the-counter supplements and prescribed drugs are vast, but it’s fair to say that certain supplements can alter the absorption, metabolism or excretion of a medicine and so affect its potency, meaning you might be getting too much or too little of a medication you need.

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As an example, the US Food and Drug Administration (FDA) states that drugs for HIV/Aids, heart disease, depression and some birth-control pills are less effective when taken with the herbal supplement, St John’s Wort. While warfarin, ginkgo bilboa and vitamin E all thin blood, so combining any of them raises the potential for internal bleeding or a stroke. And calcium, a common supplement used by many triathletes for strengthening bones, may reduce absorption of some medicines, including antibiotics.

Of course, it’s not just supplements that can interfere with medications with several foods blocking or stimulating the enzymes that break down drugs. Athletes who might wash down the cholesterol-lowering drug Lipitor with grapefruit juice, for instance, may endure muscle pain and other side-effects from statin overdose.

In short, it’s best to check with your  GP or medical practitioner before mixing prescription medication and supplements.

James Witts is a former 220 editor and specialises in writing about sports science and endurance.

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Image credit: Andrew Brookes / Cultura / Getty Images