Consumers looking to fight off colds this winter might want to consider something other than herbal supplements. Many people might already be weary of foods mislabeled as all-natural. Now, consumers should also be suspicious of herbal products contaminated or substituted with alternative plant species and fillers not listed on the labels, according to a study from the University of Guelph, Ontario published in the journal BMC Medicine.

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A DNA analysis tested the authenticity of 44 herbal products (primarily capsules) from 12 companies. Nearly half of the products were considered authentic — indicating that they contained DNA from a species that matched the main ingredient on the label — but many of those products still contained ingredients not listed on the label.

Out of the 44 total products, 30 featured product substitution. Only two out of the 12 companies tested did not feature any substitution, contamination or the inclusion of fillers. Some of these contaminants could even pose serious health risks to consumers.

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One product labeled as St. John’s wort — a plant believed to alleviate mild depression — actually contained Senna alexandrina, which is a Food and Drug Administration-approved laxative that can cause side effects from prolonged use — such as chronic diarrhea, liver damage and abdominal pain.

“Currently there are no standards for authentication of herbal products,” the study authors concluded. “Although there is considerable evidence of the health benefits of herbal medicine, the industry suffers from unethical activities by some of the manufacturers, which includes false advertising, product substitution, contamination and use of fillers.”

Researchers hope to build a DNA library that companies can use to guarantee their products are authentic.