If you're a fan of natural health products, a new study suggests using caution -- Canadian researchers have found that "the majority" of herbal products on the market contain ingredients not listed on the label.

Researchers from the University of Guelph in Ontario used DNA barcoding technology to test 44 herbal products sold by 12 companies, finding nearly 60 percent of the products contained other plants or herbs not listed on the label. More than 20 percent contained fillers such as rice, soybeans, and wheat, also not listed on the label. Only two of the companies provided authentic products without any substitutions, the researchers said.

All of the products tested are available in both the US and Canada, with some available throughout Europe, head researcher Dr. Steven Newmaster, an integrative biology professor and botanical director of the Guelph-based Biodiversity Institute of Ontario, told Relaxnews. "This is a global issue as we are now testing products from around nearly every continent."

Findings were published online Friday, October 11, in the journal BMC Medicine.

"Contamination and substitution in herbal products present considerable health risks for consumers," Newmaster said. "We found contamination in several products with plants that have known toxicity, side effects and/or negatively interact with other herbs, supplements, and medications."

One product labeled as St. John's wort, used to treat depression, contained Senna alexandrina, a plant with laxative properties. Unlike St. John's wort, it is not intended for prolonged use and can cause chronic diarrhea and even liver damage, the researchers warned.

Several herbal products contained Parthenium hysterophorus (feverfew), which can cause swelling and numbness in the mouth and nausea and can react negatively with some medications.

Unlabeled fillers such as wheat, soybeans, and rice are also a concern for people with allergies or who are seeking gluten-free products, Newmaster said.

"It's common practice in natural products to use fillers such as these, which are mixed with the active ingredients," he said. "But a consumer has a right to see all of the plant species used in producing a natural product on the list of ingredients."

Medicinal herbs now constitute the fastest-growing segment of the North American alternative medicine market, with more than 29,000 herbal substances sold, he said.

More than 1,000 companies worldwide make medicinal plant products, totalling in more than $60 billion a year. About 80 percent of people in developed countries use natural health products, including vitamins, minerals, and herbal remedies.

While Canada regulates natural health products, Newmaster added that regulators in the country face a backlog of license applications, meaning a flood of products on the market lack a full product license. Globally, regulatory problems involving natural health products continue to affect consistency and safety, he said.