Many Canadians try herbal medicines in a bid for a natural way to better health. But some of these products lack warnings that they can cause dangerous effects when mixed with pharmaceutical medications if not used properly.

New research published in BMC Medicine reveals that most over-the-counter herbal products do not contain all of the key information needed for safe use.

Researchers from the University of Leeds looked at several different brands of five commonly used remedies from pharmacies and health food stores in Britain:

• St. John's wort
• Asian ginseng
• Echinacea
• garlic
• Gingko

All of the products are generally safe, but all can cause problems in people who take certain medications or suffer from particular diseases.

So the researchers scanned the labels and boxes for key safety messages. They looked for warnings about the potential for interactions with medications, possible side-effects and information on precautions about who shouldn't use the products.

Only three of the 68 evaluated medicines contained an acceptable amount of safety information, they found. And only 13 per cent contained an information sheet.

In all, 51 of the 68 products contained none of the key safety messages about interactions and side-effects, including all of the ginkgo products, six of seven Asian ginseng products, and 20 of 21 garlic products.

St John's wort, often used as an herbal treatment for depression, can reduce the effectiveness of birth control pills. It can also affect blood thinners such as warfarin. But many of the products did not adequately warn of these effects.

Asian ginseng, often taken to increase energy or to help with nausea, is not suitable for people with diabetes, as it can affect blood-sugar levels. Ginkgo and echinacea extract can cause allergic reactions. Gingko can also interact with antiepileptic or anticonvulsant drugs

Even garlic can cause problems for some people because it thins the blood and can interfere with blood thinner medications or with drugs used to treat HIV.

"The public should be able to expect full disclosure of key safety information when they purchase herbal medicines over the counter," the authors write.

"This study has shown that, among five herbal products commonly purchased over the counter in the UK, most contained little or no information regarding what to check for in order to avoid harm before using the product."

All herbal medicines sold in Canada are subject to Health Canada's Natural Health Products Regulations. Health Canada advises that Canadian patients talk to their doctors starting any new drug or herbal remedy.