Photo

The Food and Drug Administration frequently recalls dietary supplements that are found to contain banned substances. But a new study suggests that many of these products return to store shelves months later with the same dangerous ingredients.

The findings suggest that health authorities may be fighting an uphill battle against a small number of supplement companies that repeatedly sell contaminated products. The new study, published in JAMA, the Journal of the American Medical Association, found that out of more than two dozen supplements that were pulled from shelves after they were found to contain anabolic steroids or powerful prescription drugs, roughly two-thirds were back on the market a year later with the same illicit ingredients.

Most of the supplements were marketed for weight loss, exercise and sexual enhancement, and they were sold across the country at convenience stores, in health food shops and over the Internet. They were found to contain steroids and prescription drugs like Viagra and Prozac, an antidepressant.

The study also found that several of the weight-loss products contained Sibutramine, an amphetamine-like drug that was removed from the market in the United States, Asia and Europe after a clinical trial showed it increased the risk of heart attacks and strokes.

Since 2004, about half of all F.D.A. drug recalls have involved dietary supplements found to be contaminated with banned pharmaceutical ingredients. Supplement industry trade groups say that these products are usually manufactured and sold by a few bad actors who represent the fringe of the roughly $33 billion a year supplement industry.

Under a federal law enacted two decades ago — the Dietary Supplement Health and Education Act, or Dshea — dietary supplements can be sold and marketed with little regulatory oversight. Companies do not need F.D.A. approval to sell supplements to consumers, and they do not have to provide proof that their products contain the ingredients listed on their labels.

Unlike prescription drugs, dietary supplements are considered safe until proven otherwise, and generally they are pulled from shelves only after complaints of serious injury.

In recent years, research has shown that herbal supplements such as echinacea, Ginkgo biloba and St. John’s wort are frequently mislabeled or diluted with cheap fillers like powdered rice. Last year, a nationwide study by a network of liver specialists found that the number of liver-related injuries linked to bodybuilding and weight-loss supplements was on the rise.

Jennifer Dooren, a spokeswoman for the Food and Drug Administration, said that supplement companies “are legally responsible for marketing a safe product that is not adulterated.” But because companies do not need approval to sell their products, she said, the agency cannot identify tainted supplements before they reach consumers. And even supplements that are identified as contaminated can be difficult to pursue.

“The supply chain for these products is extremely fragmented,” she said. “One product manufactured by an unknown company overseas may be sold by dozens of different distributors in the United States. The individuals and businesses selling these products generally are difficult to locate, operate out of residential homes, and distribute via the Internet, small stores and mail.”

Between January 2009 and the end of 2012, the F.D.A. recalled at least 274 dietary supplements. Many of these products returned to the market a short time later. In the new study, Dr. Pieter A. Cohen, an assistant professor at Harvard Medical School, and his colleagues purchased 27 of these products on average about one year after they had been recalled.

Although many of the previously recalled products returned to the market with slight changes to their labels or brand names, Dr. Cohen and his colleagues limited their study only to those products that returned to shelves or websites with identical packaging.

“We excluded anything that wasn’t exactly the same as it was when the F.D.A. recalled it,” he said. “But a sizable percentage of these products that are recalled, about 25 or 30 percent, are still available.”

The study found that two-thirds of the 27 supplements analyzed contained at least one unlisted anabolic steroid, prescription drug or banned substance. A majority contained the same drug or illicit ingredient that led to them being recalled by the agency. And in some cases, the products contained not only the previously identified substance, but additional drugs as well.

Twenty of the supplements that were analyzed were manufactured in the United States. Out of this group, 13 contained banned ingredients, suggesting that the problem is not limited to unscrupulous companies overseas, Dr. Cohen said.

“This is the problem with supplements: They can be introduced without any vetting at all by the F.D.A.,” he said. “These products are recalled, and then the companies keep selling the pills without making one iota of change to the product. We’re talking about very blatant flouting of the F.D.A.’s requests.”

The authors of the study called for changes to the federal law to better protect consumers from such products.

“More aggressive enforcement of the law, changes to the law to increase the F.D.A.’s enforcement powers, or both will be required if sales of these products are to be prevented in the future,” they wrote in the medical journal.

In a statement, Steve Mister, the president and chief executive of the Council for Responsible Nutrition, an industry trade group, said that “responsible manufacturers and marketers of dietary supplements” applaud the agency’s recalls.

“We have zero tolerance for this problem and welcome not only recalls,” he said, “but also criminal enforcement against companies that put consumers at risk.”

But he said that the new research “grossly misrepresents” the extent of the problem and understates the success of the F.D.A. recalls “when the law is properly enforced.”

“We urge the F.D.A. to continue using its recall authority to eradicate adulterated products and, like the responsible industry, put consumer safety first,” he said.

Dr. Cohen, who has published a number of studies identifying dangerous or illicit ingredients in dietary supplements, said that consumers should be particularly wary of products containing a mixture of herbs or ingredients — what he called “herbal cocktails.” Many of these products may be unadulterated, he said, but consumers usually cannot be sure, and often these are the sort of products that are most likely to be spiked with dangerous ingredients.

“If you want to buy herbal supplements, buy individual ingredients,” he said. “Buy echinacea or black cohosh separately. But don’t buy a mixture and don’t buy a supplement that’s sold to cause weight loss or improve your workouts. These are exactly the types of supplements that these drugs have appeared in.”