OTTAWA — Herbal medicines like St. John's Wort have long been hailed by some as safe, natural remedies for a host of ailments, but experts are now sounding alarm bells.

Many herbal remedies can produce harmful side effects when used together with prescription drugs, a Health Canada symposium was told Thursday.

"Because a product carries the word `natural,' people assume it has no side effects,'' Brigitte Zirger of the department's Therapeutic Drug Directorate explained in an interview.

"Of course that's not the case.''

For example, St. John's Wort can cause a life-threatening adverse reaction when taken by organ transplant patients who are on cyclosporin to prevent organ rejection. And the ancient medicine can also interact dangerously with warfarin, a blood-thinner used by many heart patients.

Research is difficult because natural remedies are not surrounded by the same record-keeping requirements as prescription drugs, but the overall problem of adverse drug reactions is huge.

Shiew-Mei Huang, a scientist with the U.S. Food and Drug Administration, said there are about two million adverse drug reactions in the United States annually, and 100,000 deaths.

She said one in five Americans take prescription medicines concurrent with at least one herbal product, a high-dose vitamin, or both, and 15 million are at risk for adverse reactions.

Drug regulators are debating how best to alert consumers to the risks, given the vast number of potential interactions.

"Anything could potentially cause interactions with anything,'' said Robin Marles, of the Health Canada's Natural Health Products Directorate.

"How do we narrow those down to things that are really significant?''

He said there are 42,000 natural remedies on the Canadian market, and more than half of them are combinations of different products. The market is rapidly growing, he added.

As of Jan. 1, 2004, new natural remedies sold in Canada must carry labels warning of potential risks, but Marles said it will take years before products already on the market can be brought into line with the new standards.

The symposium attracted scientists, consumer advocates, industry officials and policy-makers from across Canada and the United States.