Widely prescribed drugs including insulin, warfarin and aspirin are less effective in patients who take herbal medicines and dietary supplements.
A systematic review of relevant studies showed that popular OTC products such as St John’s wort and magnesium and calcium supplements interact with many prescribed drugs.
The researchers have recommended that GPs find out and record any OTC medicine use by patients for whom they are prescribing.
The review covered 85 studies of interactions between prescribed medicines and herbal medicines or dietary supplements, finding 882 distinct interactions.
The prescribed drugs with the greatest number of reported interactions were warfarin, insulin, aspirin, digoxin and ticlopidine.
The herbal medicines and supplements with the most interactions were St John’s wort, magnesium, calcium, iron and ginkgo – found to interact with 147, 102, 75, 71 and 51 prescribed drugs respectively.
In 42% of cases, the drug interaction was caused by the OTC product altering the pharmacokinetics of the prescribed drug.
Where the interaction meant an outright contraindication (25% of cases), the most frequent involved treatments for gastrointestinal disorders (16.4%), followed by neurological disorders (14.5%).
Professor Edzard Ernst of the University of Exeter said: “We have to become vigilant and finally agree to monitor this sector adequately. Each individual doctor can contribute to this process by routinely including questions about alternative medicine use.”