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Evidence in brief

Drug interactions and herbal remedies

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Patients should be asked routinely whether they take dietary supplements or herbal medicines, as some of these can have significant interactions with prescribed medication


Herbal remedies and dietary supplements are not licensed as medicines in the UK and are widely available without prescription. Studies from the US suggest that more than half of people with long-term conditions or cancers use herbal remedies or dietary supplements (Miller et al, 2008) and nearly one in five take these products with prescription drugs (Gardiner et al, 2006).


The Medicines and Healthcare products Regulatory Agency has produced advice for consumers on using herbal medicines safely (

National Institute for Health and Clinical Excellence (2009) guidance on depression notes that although evidence suggests that St John’s wort may be of benefit in mild or moderate depression, practitioners should:

  • Not prescribe or advise its use because of uncertainty about appropriate doses, persistence of effect, variation in the nature of preparations and potential serious interactions with other drugs; 
  • Advise people with depression of the different potencies of the preparations available and of the potential serious interactions of St John’s wort with other drugs.

New evidence

In a systematic review, Tsai et al (2012) searched from 2000 to 2010 for any scientific articles documenting interactions and contraindications between prescribed medication and herbal medicine or dietary supplements.

The review included 54 review articles, 16 clinical trials, nine observational studies and six animal studies. After excluding duplicates and products not recommended for human use, 1,491 unique interactions involving 509 drugs were identified. Overall, 100 drugs acted on the central nervous system, 90 acted on the cardiovascular system, and 75 drugs treated infection.

Most interactions were with warfarin (105 interactions), then insulin (41 interactions), aspirin (36 interactions) and digoxin (32 interactions). The herbal remedies with the greatest number of interactions with drugs were St John’s wort (147 interactions), ginkgo (51 interactions) and kava (41 interactions). The dietary supplements with the most drug interactions were magnesium (102 interactions), calcium (75 interactions), iron (71 interactions), vitamin A (43 interactions) and melatonin (30 interactions).

Many of the supplements with high numbers of interactions seemed to have general class effects for interactions classified as severe. For example, digitalis had severe interactions with several thiazide and thiazide-like diuretics and kava had interactions with sedating antihistamines and benzodiazepines. St John’s wort had interactions across a wide range of drug classes including antidepressants, chemotherapy drugs, sex hormones and cardiovascular drugs.

The authors noted that health professionals may need to be aware of the most common interactions to prevent adverse events, especially in patients taking drugs acting on the central nervous or cardiovascular systems.

Adapted from Eyes on Evidence (Ju

ne, 2013), a bulletin produced by the National Institute for Health and Care Excellence. Reproduced with permission

Lynne Kincaid is medical writer, National Institute for Health and Care Excellence

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