A DNA test of herbal products has found that most of them contained cheaper fillers and plant ingredients not listed on the label, some of which pose "serious health risks."

Researchers at the University of Guelph used DNA barcode testing to test 44 herbal products from 12 companies. DNA barcoding uses a short sequence of DNA from a standard segment in plants to identify the species rapidly and accurately.

"Product substitution occurred in 20/44 of the products tested and only 2/12 companies had products without any substitution, contamination or fillers," Steven Newmaster, an integrative biology professor at the University of Guelph and his co-authors concluded in Friday's issue of the journal BMC Medicine.

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what do Canadians think about natural health products? (Duk Han Lee/CBC)

"Some of the contaminants we found pose serious health risks to consumers."

The researchers would not name the products. 

The World Health Organization calls adulteration of herbal products a threat to consumer safety.

In the study, one product labelled as St. John's wort contained Senna alexandrina, a plant with laxative properties. S. alexandrina isn't meant for prolonged use as it can cause chronic diarrhea and liver damage, and a study suggests it can interact with immune cells in the colon.

Several herbal products contained Parthenium hysterophorus (feverfew), which can cause swelling and numbness in the mouth, oral ulcers and nausea. It also reacts with medications metabolized by the liver and may increase the risk of bleeding, especially if taken with blood-thinning medications, the researchers said.

One ginkgo product was contaminated with Juglans nigra (black walnut), which could be dangerous for people with nut allergies.

Most herbal products 'poor quality'

Newmaster's team said the target crop may have been harvested along with walnut leaves that contain a toxic compound.

The tests identified several potential fillers, including rice, soybean and grasses such as wheat that could pose a health concern for people allergic to those plants and for consumers seeking gluten-free products.

"Most of the herbal products tested were of poor quality, including considerable product substitution, contamination and use of fillers," the study's authors concluded.

Medicinal herbs are the fastest growing segment of the North American alternative medicine market, previous studies suggest.

But there are currently no best practices for identifying the species of various ingredients used in herbal products, the researchers said.

They're proposing that the herbal industry voluntarily use DNA barcoding to authenticate herbal products by testing raw materials to gain consumer confidence.

Canada has regulated natural health products since 2004. 

In a statement, Health Canada said it will take action if products pose a risk to customers. "Health Canada recommends that Canadians buy and use only authorized natural health products from sources they trust. All retailers have the responsibility to sell compliant products," a spokesman for the department said in an email.

A spokeswoman for the Canadian Health Food Association, which represents manufacturers, said the industry is happy to look at DNA barcoding once it is validated.

With files from CBC's Kim Brunhuber