Over 150 of the tested supplements contained more than one unapproved ingredient.
The most common pharmaceutical ingredients detected in adulterated weight loss products were sibutramine, which was removed from the United States market in 2010 due to cardiovascular risks, and the laxative phenolphthalein.
The FDA explicitly warns that supplements aren't a replacement for either over-the-counter or prescription medications, and should not be viewed as a way to treat or prevent disease.
Over half of American adults take dietary supplements every day, from vitamins and minerals to amino acids and botanicals.
The greatest number of products found to contain hidden ingredients were reported in 2009, when two large recalls together named 99 products.
They also noted that another study found that 23,000 emergency visits per year are due to supplements.
The research was based on a Food and Drug Administration (FDA) database that identifies "tainted" supps, those which contain ingredients not actually listed on the label. Most of the adulterated supplements were marketed for sexual enhancement, weight loss or muscle building. They are not meant to treat or prevent disease and are not subject to premarket safety and efficacy testing. Among them, dapoxetine, an antidepressant that is not approved in the United States, and sibutramine, which was included in some weight-loss supplements but was banned from the US market in 2010 because of cardiovascular risks.
"In this quality improvement study, analysis of the US Food and Drug Administration warnings from 2007 through 2016 showed that unapproved pharmaceutical ingredients were identified in 776 dietary supplements, and these products were commonly marketed for sexual enhancement, weight loss, or muscle building".
And among muscle-building supplements, synthetic steroids or steroid-like ingredients were the cause for concern almost 90 percent of the time, the researchers said.
"Only 360 of 746 (48 per cent) were recalled, leaving the majority of adulterated supplements, more than 350 products, available for sale", Cohen said.
Most supplement consumers wrongly believe that the products they are purchasing are regulated or vetted in some way. Another would give the FDA tougher enforcement tools so that a product's registration could be withdrawn if found to be adulterated.
FDA spokeswoman Lindsay Haake wrote in an email that "the FDA recognizes the seriousness of this problem and continues to act within its resources and authorities to address this problem as best it can". "How could it be that our premier public health agency spends the time and money to detect these hidden ingredients and then doesn't take the next obvious step, which is to ensure that they are removed from the marketplace?" Of these products, 19 (67.9%) were reported to contain new unapproved ingredients in the second or third warning, consistent with the assumption that the FDA found the product to be adulterated a few times. The FDA has also found that distributors often re-label products to evade detection.
The researchers say the adulterants have the potential to cause adverse health effects "owing to accidental misuse, overuse, or interaction with other medications, underlying health conditions, or other pharmaceuticals within the supplement".