Dietary supplements containing banned drugs often remain on sale long after they have been recalled by the Food and Drug Administration (FDA), doctors at Harvard Medical School have found.

The FDA has identified more than 400 supplement brands that contain banned pharmaceutical ingredients, and has issued a recall for 70 percent of them, according to Reuters.

The study involved 27 supplements recalled between 2009 and 2012. They were marketed for sports enhancement, weight loss and sexual enhancement. Two-thirds were manufactured in the United States, the article notes. The researchers purchased the supplements from manufacturer websites at least eight months and up to four years after they were recalled.

After testing the supplements’ chemical makeup, the researchers found 18 of the supplements still contained a banned pharmaceutical ingredient. These included a weight loss drug linked to heart attack and stroke called sibutramine, and a laxative called phenolphthalein that is being removed from many markets because of a potential link to cancer. The findings appear in JAMA.

“There’s no question that these supplements that contain pharmaceuticals are not allowed to be sold, there are clear-cut laws,” lead author Dr. Pieter A. Cohen told Reuters.

The FDA noted the supply chain for dietary supplements is extremely fragmented. “One product manufactured by an unknown company overseas may be sold by dozens of different distributors in the United States,” the agency said in a statement. “The individuals and businesses selling these products generally are difficult to locate, operate out of residential homes, and distribute via Internet, small stores, and mail. Products are shipped through the international mail facilities and are often misdeclared as unrelated goods to avoid detection. Even after recall and enforcement action against one major distributor, the product may continue to be widely sold.”