Co-ingestion of herbal medicines and warfarin

Abstract

Background: A large proportion of patients use herbal remedies with a potential to interact with prescribed drugs. Such interactions can be dangerous, particularly if the therapeutic window of the prescribed drug is small, as with warfarin.

Aims: Our aim was to estimate the prevalence of the use of herbal medicines by patients taking warfarin (co-ingestion).

Design of study: Postal questionnaire.

Setting: General practices in the South West of England.

Method: Thirty-five general practices in Devon and Somerset identified 2600 patients taking warfarin and sent postal questionnaires to them.

Results: One thousand, three hundred and sixty usable responses were received (response rate = 54.2%). One or more of the specified herbal remedies thought to interact with warfarin were taken by 8.8% of all patients. Complementary or homeopathic treatments not specified in the survey questionnaire were taken by 14.3% of responders. Overall, 19.2% of responders were taking one or more such medicines. The use of herbal medicines had not been discussed with a conventional healthcare professional by 92.2% of patients. Twenty-eight point three per cent of responders thought that herbal medicines might or definitely could interfere with other drugs prescribed by their doctor, however, patients taking any non-prescribed medication were less likely to believe this (chi2 = 20, degrees of freedom = 1, P<0.001).

Conclusion: A substantial proportion of patients taking warfarin in southwest England self-medicate with both herbal medicines that are thought to interact with warfarin and with others of unknown effect, usually without informing their healthcare team. Patients have a responsibility to mention such non-prescribed medication to their general practitioners, and general practitioners also have a responsibility to ask whether such co-ingestion is occurring.