Taking a daily multivitamin or fish oil pill is a common ritual. In fact, Americans spend more than $30 billion a year on dietary supplements.

But a growing body of evidence suggests that popular vitamins and supplements may not always contain what their labels claim, and potentially dangerous ingredients can get into the pills. A recent October 2018 report by California Department of Public Health researchers, published in the journal JAMA Network Open, revealed that more than 700 supplements on the market included unlabeled ingredients found in pharmaceutical drugs, and less than half received voluntary recalls from health authorities. “That’s just the tip of the iceberg,” says Dr. Pieter Cohen, a Cambridge Health Alliance physician who researches supplements.

Here’s everything you need to know about dietary supplements, and how to take them safely.

Do I Need to Take a Supplement?

Estimates suggest more than half of all Americans take dietary supplements, but many may not need them. If you’re eating a healthy diet and don’t have an underlying health condition that prevents you from absorbing nutrients from food, you probably don’t need extra supplements. “For most healthy people who are not on a very restrictive diet, like strict vegan, there is no need for vitamins with only two common exceptions,” Cohen says.

Women of childbearing age should take a folic acid supplement, and post-menopausal women should take calcium supplements if they’re not getting enough from their diet, Cohen says. There are also people at greater risk of developing nutritional deficiencies who can benefit. For example, women who have heavy periods, people with cancer, and people who frequently donate blood are all more likely to be iron deficient, according to the National Institutes of Health (NIH). In this case, iron supplements may be recommended by a doctor.

“The bottom line is that people should talk with their health care provider for advice,” says Carol Haggans, a nutrition scientist and consultant to the Office of Dietary Supplements at the NIH. That may seem like obvious advice, but estimates suggest that only about a quarter of Americans who take supplements are taking them at the recommendation of a physician.

How Are Vitamins and Supplements Regulated?

Dietary supplements and vitamins are regulated by the U.S. Food and Drug Administration (FDA), but they are not required to be tested for safety or efficacy before they are sold.

“There is zero approval process,” Cohen says.

The FDA does provide guidance documents for manufacturers on the safety requirements for supplements. For example, supplements cannot claim that they prevent or cure disease. Instead, labels and advertising can only make general statements about supplements’ ability to maintain the health and functioning of the body. A supplement can claim it boosts immune system function, but it cannot say it fights infection.

“Companies are supposed to have data to support those claims,” says Dr. Joshua M. Sharfstein of the Johns Hopkins Bloomberg School of Public Health. “But [that] data is not reviewed by the FDA prior to sale. In fact, the FDA doesn’t really do anything prior to sale.”

How Do Extra Ingredients Get into Supplements?

In the new study, researchers reviewed a database of adulterated supplements maintained by the FDA and found unapproved drugs, like steroids, antidepressants, and Viagra, were discovered in 746 dietary supplements between 2007 and 2016. More than half the products were not removed from store shelves.

That’s a major problem, Sharfstein says. “Someone may be told by their doctor that they’re not healthy enough to take Viagra and then they go out and buy a supplement, and they’re getting unapproved Viagra,” he says, adding that the study “demonstrates that the FDA does not really have control over the market.”

The new findings are not surprising to Cohen, who wrote an accompanying commentary to the study.

In 2014, Cohen and a team of researchers similarly found that even after FDA recalls, products remain on the market, with most still containing unapproved drugs. The new study, he says, magnifies the extent of the issue. A previous study found that an estimated 23,000 ER visits each year are attributed to dietary supplements.

It’s unclear how unlabeled ingredients and pharmaceutical drug products end up in supplements, but Cohen has a theory. He believes it’s because companies are allowed to “legally advertise supplements as though they improve your health even if there’s no evidence that they actually work” as long as the words “cure” or “treat” aren’t used, he says.

To meet those claims, Cohen speculates that companies sometimes spike their products deliberately. “It creates a perverse incentive to try to meet what you’re allowed to advertise,” Cohen says. This hasn’t been proven, but the FDA says it is reviewing the new study findings.

In the recent study, muscle building, sex enhancement, and diet supplements accounted for 98 percent of the adulterated products. “People really want to find something that helps them in those three areas,” Sharfstein says. “The easiest way for companies to deliver on that is to spike their products with illegal drugs. If you want to sell a product for erectile dysfunction, it’s going to work better if you put Viagra in there.”

What Can Be Done About Adulterated Supplements?

If a product is deemed unsafe or adulterated, the FDA can issue a warning to companies and notify the public. But there’s no real system for detecting these issues, Cohen says.

Because anyone can create and sell a supplement, there’s no way to track what’s on the market. If a manufacturer notifies the FDA of adverse effects—companies are only required to notify when serious issues like hospitalization or death occur, Cohen says—or consumers file a complaint directly with the agency, the FDA tests the supplement. But according to the new study, the FDA was aware of adulterated products and still didn’t act. “The FDA has a lot of options if a company refuses to issue a recall. But what we can see by this study is that they’re simply not doing it,” Cohen says. “Honestly it’s completely inexplicable.”

FDA spokesperson Jeremy Kahn says the FDA is aware of the study and is in the process of reviewing its findings. “The FDA is committed to doing everything within its resources and authorities to identify and remove unsafe products from the market,” Kahn said in a statement, but added that the “FDA does not have mandatory recall authority for drugs.”

I Still Want or Need to Take a Daily Supplement. How Do I Know I’m Buying a Safe Product?

First and foremost, talk to a doctor about whether you need a supplement. If you’re looking up information about supplements online, pay attention to the source of the information. Is it coming from a medically reputable group?

If you choose to try a supplement, “stay away from any supplement that makes claims that it will work to improve your health,” Cohen says, since these claims are not vetted. Also, avoid weight loss, sexual enhancement, and muscle-building supplements because of the high risk of adulterated products.

Cohen also recommends sticking to single-ingredient supplements, like vitamin D, because they’re much less likely to be adulterated. “It’s easier to hide pharmaceuticals from regulators in a complicated mix of ingredients,” he says.

Also keep in mind that unless you have trouble absorbing nutrients, you can probably get adequate amounts of nutrients through a healthy diet. That means you need to eat a lot of vegetables and fruit.

Ultimately, Sharfstein says the best and arguably only way to ensure a supplement is safe is with a new law: “I don’t think the FDA has the capacity to solve this problem effectively without new legislation.”

Update: An earlier version of this story incorrectly stated how much Americans spend on supplements. The figure is more than $30 billion a year.