Dietary supplements are wildly popular. About 114 million Americans — roughly half the adult population — take at least one supplement, running up a collective tab of over $28 billion in 2010 alone. It’s easy to see why supplements are such big sellers. The public has a legitimate desire for good health, and the supplement industry has a strong desire for good sales.

Medications are regulated by the FDA. Before a prescription or over-the-counter drug can be sold in the United States, the manufacturer must submit data supporting its safety and efficacy, and after the medication is approved, the FDA continues to monitor adverse reactions. Even with all these safeguards, problems still occur, prompting the FDA to withdraw many medications and to require strong warning labels on others.

In 1994, the Dietary Supplement Health and Education Act sharply restricted the FDA’s ability to regulate products marketed as “dietary supplements,” even though most people buy them for health, not nutrition. Manufacturers can sell these products without submitting evidence of their purity, potency, safety, or efficacy.

For most claims made on product labels, the law does not require evidence that the claim is accurate or truthful. In fact, the FDA’s first opportunity to weigh in comes only after a product is marketed, when it can take action against products that are adulterated, misbranded, or likely to produce injury or illness. Since nearly all supplements are used without medical supervision or monitoring, most of the estimated 50,000 adverse reactions that occur in the United States each year go unreported. It’s a situation that Harvard’s Dr. Peter Cohen has called “American roulette.”

How do we really know?

If a supplement’s label or ads won’t give you reliable information, how can you find out if a supplement can help — or, for that matter, hurt? Although it’s a slow process, careful, objective medical studies provide the guidance that counts.

In most cases, scientific investigations of supplements start with simple observational studies, in which researchers compare the health status of folks who take a particular supplement with the health of people who don’t take the supplement. It’s an important effort, but the results don’t always hold up. So the next step is to conduct randomized clinical trials, in which volunteers are assigned by lot to take either the supplement or an identical-looking placebo (“dummy pill”) while researchers track their health. In the best studies, neither the volunteers nor the researchers know who is getting the real thing until the code is broken at the end of the trial.