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Canadians wary of antidepressants may rely on St. John's wort, an age-old herbal remedy, to keep mood disorders at bay. But while the yellow-flowering plant is potent enough to relieve mild to moderate depression, it is also the herbal medicine most likely to interfere with pharmaceutical drugs, researchers have found.

St. John's wort is documented to interact with 147 different medications, including the blood-thinner warfarin and cholesterol-lowering statins, according to a review of 85 studies published in the International Journal of Clinical Practice.

Next in the list of herbal and dietary supplements causing the most drug interactions are magnesium, calcium, iron and ginkgo biloba, which were found to interact with 102, 75, 71 and 51 different drugs, respectively.

Interactions with herbs, vitamins and minerals may amplify or reduce a drug's effects. Of the 1,490 interactions captured in the study, researchers identified that 17 per cent posed a major health risk, such as excessive bleeding, a hypertensive crisis or potential coma.

Drug interactions with herbal remedies and dietary supplements are "tremendously under-reported," says Simon Pickard, co-author of the study and associate professor of pharmacy at the University of Illinois.

Earlier research suggests that less than a third of patients inform health- care providers when they're on herbal or dietary supplements. If patients have an adverse effect while taking both pharmaceutical drugs and natural remedies, Pickard says, "they may not make the connection."

The study, led by Hsiang-Wen Lin at the China Medical University's College of Pharmacy in Taiwan, used data from clinical trials, observational studies and review articles to assess the breadth of interactions and contraindications associated with herbal and dietary supplements, Pickard says.

Warfarin had by far the largest number of reported interactions with supplements, followed by insulin, Aspirin, digoxin (a drug used to slow heart rate and treat congestive heart failure) and ticlopidine (an anti-platelet drug).

Researchers also looked at herbal and dietary supplements documented to be harmful for patients with specific conditions. Flaxseed, echinacea (used to ward off colds) and yohimbe (a folk remedy for sexual dysfunction) had the highest number of documented contraindications. Flaxseed, for example, was contraindicated for patients with gastrointestinal disorders, high triglyceride levels or prostate cancer.

The study offers critical information to health-care professionals, Pickard says. "We were able to hone in on those disease states where we know the most about interactions and contraindications," he says. "We should be more aware with patients that have these conditions that they will potentially encounter problems if they are using [certain] herbs or dietary supplements."

Patients with life-threatening diseases, such as cancer, often turn to herbal remedies because "they're desperate for a solution," he points out. Nevertheless, Pickard urges patients to be upfront with health-care providers about any plant extracts, vitamins, minerals or amino acids they're taking. Many pharmacists are knowledgeable about herbal and dietary supplements, he says, "but we can't identify drug-herbal interactions or contraindications if a health-care professional is not aware that the patient is on them."

Advocates of traditional medicines might argue that herbs have been safely used for centuries and pose little risk if taken under the guidance of a clinically trained herbalist. Historically, however, patients did not combine herbs with the powerful pharmaceutical drugs available today.

Research on herbs and drug interactions has not caught up with the public's casual approach to trying out supplements from the health-food store. In an editorial accompanying the study, Edzard Ernst, a specialist in complementary medicine at the University of Exeter in Britain, notes the herb and drug interactions documented by the study could be just "the tip of the iceberg."

He underscores the need for evidence-based information about herbal and dietary supplements and their interactions with pharmaceutical products. As long as reliable data remains scarce, patients looking for health information online will be "bombarded with commercially driven misinformation" about alternative medicine, he writes in an e-mail.

These products, he warned, can be "outright dangerous."