Co-ingestion of herbal medicines and warfarin

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.

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

Postal questionnaire.

General practices in the South West of England.

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

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).

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.