The presence of prescription medicines, often at unknown concentrations, means these supplements are essentially "unapproved drugs," which could be harmful to the health of those who take them, according to the study authors.
"These products have the potential to cause severe adverse health effects owing to accidental misuse, overuse, or interaction with other medications, underlying health conditions, or other drugs within the same dietary supplement," wrote the authors, led by Madhur Kumar of the California Department of Public Health Food and Drug Branch.
More than 50% of adults use dietary supplements, a $35 billion industry, notes the study.
The greatest number of products found to contain hidden ingredients were reported in 2009 when two large recalls together named 99 products. Otherwise, 443 of the total 776 products were reported from 2012 to 2016. In the overwhelming majority of cases (97%) the unapproved pharmaceutical ingredients were not declared on the label of the supplements.
Most of the adulterated products, about 45%, were marketed for sexual enhancement, weight loss (about 41%) or muscle building (12%). Drugs found in sexual enhancement products included sildenafil, tadalafil and vardenafil, all active ingredients in prescription medications intended for erectile dysfunction, which, when overused, can cause serious damage to the blood vessels.
"As the dietary supplement industry continues to grow in the United States, it is essential to further address this significant public health issue," the study authors concluded.
'Consumers' health at risk'
Cohen, an associate professor of medicine at Harvard Medical School, believes that "more than FDA action will be required to ensure that all adulterated supplements are effectively and swiftly removed from the market."
CNN has reached out to FDA for comment.
Cohen suggests Congress reform the Dietary Supplement Health and Education Act of 1994 by requiring companies to register supplements with the FDA prior to sale and by providing the FDA with more effective enforcement tools such as immediately revoking an adulterated product's registration.
In the meantime, the FDA's "failure to aggressively use all available tools to remove pharmaceutically adulterated supplements from commerce leaves consumers' health at risk," Cohen wrote.