About two-thirds of people who use herbs don't follow scientific guidelines before taking them, a U.S. study suggests.

Sales of herbal supplements more than doubled between 1994 and 2004 — to $18.8 billion US from $8.8 billion US — but there are minimal federal regulations, researchers said.

In a survey of more than 2,000 adults conducted for Health Canada in 2005, 71 per cent reported they regularly take vitamins and minerals, herbal products and homeopathic medicines.

There are concerns that herbal supplements can have side-effects and interact negatively with therapeutic drugs.

Physicians should therefore ask patients about their use of herbs at every visit and admission to hospital, and inform patients about potential benefits and harm, the study's authors said.

"Health care professionals should take a proactive role, and public health policies should disseminate evidence-based information regarding consumption of herbal products," lead author Dr. Aditya Bardia, a resident in internal medicine at the Mayo Clinic in Rochester, Minn., and her colleagues concluded in the May issue of the Mayo Clinic Proceedings.

The alternative health study used information from a national survey of more than 5,700 adults in the U.S. who said they had consumed herbs in the previous 12 months.

Of the 609 people who said they were taking a single herb to treat a specific health condition, there was a known scientific indication for the herbal preparation in one-third of cases, the team reported in the May issue of Mayo Clinic Proceedings.

More needs to be learned about the clinical indications of individual herbs and their efficacy, Bardia said.

Health Canada's guidelines for natural health products aim to clear up confusion about conflicting health claims and warnings about mixing natural products with other medicines. The department's Natural Health Products Directorate regulates the industry.