Combining anticoagulant drugs with herbal medicines is risky, report says

A review of reported interactions between herbal and conventional drugs has found more than 100 cases but warns that more research is urgently needed.

“Herb-drug interactions occur but are under-researched. In many cases there is no plausible mechanism to explain the observed phenomena and causality is uncertain,” says the report in the British Journal of Clinical Pharmacology (2001;52:587-95).

The aim of the review, by Professor Edzard Ernst of the Department of Complementary Medicine at Exeter University and Adriane Fugh-Berman of George Washington University School of Medicine, Washington, DC, was to assess published clinical evidence on interactions between herbal and conventional drugs.

Four electronic databases were searched for case reports, case series, or clinical trials of interactions. Ten major manufacturers of herbal products were asked for any data on interactions, and eight experts and 24 organisations related to medical herbalism were invited to contribute further material.

A 10 point scoring system for interaction probability was used to validate the data. Criteria looked for included adequate details of patient history, concurrent diseases, and the adverse event itself.

A total of 108 cases of suspected interactions were found, 69% of which were classed as “unable to be evaluated,” 13% as “well-documented,” and 19% as possible interactions.

Warfarin was the most common drug, with 18 cases, and St John's wort was the most common herb. Other herbs included dong quai, ginseng, garlic, ginkgo, and kava.

“St John's wort was the herb most commonly implicated in interactions, and a total of 85 cases have been reported, of which 54 cases or 63.5% were with cyclosporin,” says the report. “Other reports involving St John's wort included 12 cases of interactions with oral contraceptives, seven with warfarin, nine with antidepressants, and one each with phenprocoumon, theophylline, and loperamide. The risks of combining St John's wort with drugs or anticoagulant drugs with herbal medicines should be publicised further.”

It adds, “There is reasonable documentation of interactions between coumarin anticoagulants and St John's wort, danshen, dong quai, ginseng, and ginkgo. Most of these case reports are probably not true interactions but result from additive anticoagulant affects. Patients on coumarin anticoagulants should be specifically advised to avoid taking herbal medicines or to have their INR [international normalised ratio] measured within two weeks of starting the product.”

The report says that inadequate reporting makes it difficult to determine whether a herb-drug interaction has occurred, and adds, “Authors should be required to document all relevant information. The adverse event should be clearly described, alternative explanations should be explored and a re-challenge should be considered. Even well-documented case reports can only serve as a critical early warning system.”