A tough pill to swallow: Are dietary supplements safe?

The dietary supplement industry brings in billions of dollars each year, with an estimated 90,000 products on the market, including vitamins, minerals, herbs or other botanicals, amino acids, probiotics, or other substances in pill, capsule, tablet, or liquid form. About half of American adults take at least one dietary supplement for health or wellness, or to fill nutrition gaps. But are these supplements safe?

If you answered “Yes,” you’re not alone. A 2015 Consumer Reports survey found that most people think that dietary supplements are vetted for safety by the U.S. Food and Drug Administration (FDA). Unfortunately, this isn’t true. Unlike prescription or over-the-counter drugs, which must be approved by the FDA before they can be marketed, the FDA is not authorized to review dietary supplements for safety and effectiveness before they are sold. The FDA can take action through warnings or recalls — often voluntary — if they receive reports that a supplement already on the market is causing harm, but this can take several years, if it happens at all, and may not be effective.

One of the biggest concerns about supplement safety is adulteration with ingredients that aren’t listed on the label, including ingredients that may be toxic at high doses. In some cases, a less expensive ingredient is used instead of a more expensive ingredient listed on the label. In other cases, the supplement is illegally adulterated with pharmaceutical drugs — sometimes drugs that have been pulled from the market because of safety concerns, or have never been approved in the first place. This is most common with botanical or “natural” supplements.

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The FDA has raised concerns about three certain categories of supplements, including those targeting weight loss, sports performance, sexual function, or cognitive health. A 2015 study lead by the FDA and the Centers for Disease Control and Prevention found that, among adults, 25 percent of emergency room visits for adverse effects from dietary supplements were due to weight loss supplements, with cardiac symptoms being the primary complaint.

Dr. Pieter Cohen, an associate professor of medicine at Harvard Medical School, says it’s not clear why these supplement categories are rife with adulteration.

“One hypothesis is that permitting supplements to be legally sold as if they can help a consumer lose weight creates a perverse incentive for manufacturers to adulterate the product with pharmaceutical drugs,” he says. “Initially, it’s easy to sell a product for weight loss, but consumers will rapidly lose interest when it doesn’t work. The claims, however, create an incentive to maintain sales by adulterating the product with pharmaceutical weight loss drugs that pose risks to consumers.”

In a 2020 study, Cohen and his research team analyzed 10 over-the-counter dietary supplements marketed to improve memory and cognitive function. They detected five unapproved drugs in the supplements, in some cases in amounts four times higher than pharmaceutical doses. In 2015, Cohen’s team discovered an untested-in-humans stimulant similar to methamphetamine in a dozen supplements sold in the US for weight loss, improved brain health, and improved athletic performance.

A 2018 analysis of the FDA database of pharmaceutically adulterated supplements found that of 746 supplements identified by the FDA, only 360 were issued voluntary recalls, and there’s no guarantee that the manufacturers complied.

Several independent organizations — notably U.S. Pharmacopeia (USP), ConsumerLab and NSF International — offer quality testing. Their seals of approval mean that the supplement was properly manufactured, contains the ingredients listed on the label, and doesn’t contain harmful levels of contaminants. That can give you confidence that the product isn’t adulterated, but it doesn’t guarantee that a product is safe or effective.

“I always suggest purchasing products that invest in third-party certification,” says Seattle registered dietitian Ginger Hultin, MS, RD, a spokesperson for the Academy of Nutrition and Dietetics. “So many supplements come back with levels of ingredients that are above or below what’s listed in the label that at least these companies help supplement brands offer that type of protection.”

Cohen says there are some straightforward first steps the FDA could take to enhance the quality and safety of supplements, including creating a mandatory registry of all supplement products that links every product to a specific product code. The FDA could allow registry access only to products listing legal supplement ingredients on the label — and immediately remove a product if it’s found to be hazardous or adulterated. Retailers could then be required to only sell registered supplements.

“Creating a mandatory registry with strong regulatory safeguards could move us toward an environment where a consumer purchasing a supplement online or in brick-and-mortar stores could be assured of its legality and safety,” he says.

Hultin points out that any dietary supplement — even if it’s not adulterated — has the potential to be harmful to certain people.

“Even vitamin C, which is water soluble and is eliminated from the body when we take high amounts, can be harmful,” she says. “At high levels, it can cause digestive issues including diarrhea and it’s also not safe for people at risk of or who have kidney stones. Before taking absolutely any supplement, people should make sure that it’s not interacting with any of their medications or other supplements and that it’s safe for them based on their unique needs and medical history.”

Environmental Nutrition is an independent newsletter written by experts on health and nutrition.