With the growing interest in natural and alternative health supplements, here’s a warning for consumers. A study out of the University of Guelph finds that the majority of herbal products on the market contain ingredients not listed on the label, with most companies substituting cheaper alternatives and using fillers.

The study in the journal BMC Medicine, used DNA barcoding technology to test 44 herbal products sold by 12 companies.

Only two of the companies provided authentic products without substitutions, contaminants or fillers.

Nearly 60 per cent of the herbal products contained plant species not listed on the label. Researchers also found product substitution in 32 per cent of the samples, and fillers in 20 percent. In those cases, substances like rice, soybeans and wheat were added but not listed. The study authors say this can pose serious health risks because some of the plants used have known toxicity and side effects, or they react negatively with other herbs, supplements and medications. Unlabelled fillers are also a concern for people with allergies or those wanting gluten-free products.

For instance, one product labelled as St. John’s Wort contained Senna alexandrina, a laxative that’s not intended for prolonged use, since it can cause chronic diarrhea and liver damage.

The global market for medicinal herbs is worth $60 B a year with 29,000 products sold in North America.