If you take a trip down “memory” lane—in this case the aisle at your local pharmacy or vitamin store—you’ll see lots of products that claim to improve your brain function. Senator Claire McCaskill, a Missouri Democrat, recently called for an investigation of many of these products, as well as the advertising practices the companies use to promote them. A new study helps explain why McCaskill and several other lawmakers are so concerned.

Researchers in Massachusetts and Mississippi bought 23 dietary supplements from two of the largest U.S. supplement retailers, GNC and Vitamin Shoppe, that were labeled as containing the ingredient vinpocetine. That’s a drug prescribed in China, Germany, and Russia for stroke and cognitive impairment, though it has not been approved in the U.S. and its effectiveness and safety are questionable. Vinpocetine can also cause side effects such as dangerously low blood pressure, headaches, and flushing.

While vinpocetine can be synthesized from an alkaloid found in the leaves of the Vinca minor plant (aka lesser periwinkle), it is not a naturally occurring ingredient and therefore should not be sold as a dietary supplement in the U.S.  

Yet the researchers detected vinpocetine in 17 of the 23 supplements they tested, in amounts ranging from 0.3 milligrams to 32 mg. The prescription doses in countries where vinpocetine is used range from 5 mg to 40 mg.

No vinpocetine was found in the other six products, even though their labels claimed the ingredient was in them. And only six that contained vinpocetine had the amount they claimed. The researchers said that nearly 275 other products sold in the U.S. list vinpocetine on the label. And the products are sold not only in GNC and Vitamin Shoppe but other stores as well.

"The FDA should not allow unapproved drugs to be sold as dietary supplements, but the agency has done nothing to intervene in the sale of these and many other products,” says Pieter Cohen, M.D., an internist at Harvard Medical School and one of the authors of the current study. “It’s bad enough when the public is duped into spending money on expensive products that are no better than a sugar placebo, but it’s even worse when the product is actually an unapproved prescription drug with important physiologic effects.”

Vinpocetine is hardly the only dietary supplement spiked with prescription drugs or other unlisted ingredients. In fact, the FDA maintains a “tainted supplements” database that gets added to almost weekly. And the FDA itself admits that the products listed there likely represent just the tip of the iceberg.

“This is not the only bad apple in the barrel but just the latest in a lengthy list of bad apples," says Consumer Reports chief medical adviser Marvin M. Lipman, M.D. "All dietary supplements should be viewed with great caution and used at one’s own risk," Lipman says.