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WASHINGTON -- Herbal products and supplements are frequently mislabeled and may contain unlisted ingredients that are harmful to the liver, researchers said here.

Of 272 herbal dietary supplements analyzed, 51% had inaccurate labels in which some or all of the listed ingredients were not detected in the product, Victor Navarro, MD, of the department of transplantation at the Einstein Healthcare Network, in Philadelphia, and colleagues found.

"Herbal supplements are a common cause of liver injury," Navarro said at a press briefing at the . "There are lots of products that are difficult to identify what they are and what they're used for."

A group of investigators known as the Drug-Induced Liver Injury Network, supported by the National Institute of Diabetes and Digestive and Kidney Diseases, collects cases of drug-induced liver injury; in 20% of cases, the injury was caused by dietary supplements, said Navarro, a member of the network. Many of the products are sold as body-building supplements or weight-loss supplements.

"A lot of the literature tells us supplements can be mislabeled or adulterated" -- the latter occurring when something is secretly included to support the purpose for which it is being marketed, such as for sexual enhancement, he added.

Members of the 14-year-old network, which comprises six clinical centers and a data coordinating center, documented drug-induced liver injuries in 1,775 patients from 2003 to 2015. Of those, 375 reported taking at least one dietary supplement, and 101 patients contributed 337 supplement samples for analysis.

Of those samples, 272 had labels and were suitable for chemical analysis. Within that group, 96 products, taken by a total of 71 patients, were determined to be causes of liver injury.

The investigators then analyzed the supplements using high-pressure liquid chromatography and mass spectrometry. Some were found to contain hepatotoxins including anabolic steroids or pharmaceuticals, Navarro said. "We could look at the cases and determine that the actual injury came from the unlabeled ingredient ... The next step is to look more carefully at all of these cases [and] continue our chemical analysis."

Dietary supplements are regulated very differently from drugs, and they are not tested for safety, he pointed out. "If you look at the definition of a dietary supplement, it implies there is a deficiency in the diet." While he and his colleagues can't name a specific supplement to avoid, "When a provider is asked which supplements someone should take, if there's no dietary deficiency, there is no reason to recommend that a patient should take supplements," said Navarro.

All of these injury cases have been reported to the FDA MedWatch database, he said. "Many products we have identified as having these ingredients are not even marketed any more, and that could [happen over] a matter of months ... We're at the leading edge of this; we're going to have to tighten that association [between the products and the injuries] and that's going to be a challenge. We've identified some products we're convinced cause liver injury [but] right now it's very circumstantial."

One positive outcome of the study is that it highlights the fact that these herbal products contain ingredients that are unknown to the patients taking them, said Norah Turreault, MD, of the University of California San Francisco, who moderated the session where the research was presented. However, the researchers are being cautious because "establishing what's in them that's related to hepatotoxicity is where they're really working to build a stronger causality argument ... But some awareness that there are things in the products that are not labeled is an important message to get out."

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