Dietary supplements: what the evidence says

A supplement is not a drug, and that difference cuts both ways

Under the Dietary Supplement Health and Education Act of 1994, the Food and Drug Administration regulates dietary supplements under a different set of rules from those that cover drugs. Manufacturers and distributors are responsible for evaluating safety and labeling before a product is sold. The FDA can act against an adulterated or misbranded product after it is on the market. That system is not the premarket approval process used for prescription drugs. A disclaimer on a bottle does not create permission to claim that the product treats, cures, or prevents a disease. A structure-or-function sentence, if one is legally used, still needs substantiation and the labeling the law requires. This website is not reviewing any commercial product, and it is not a place to buy one.

The Office of Dietary Supplements puts the practical point simply: people should get most nutrients from food. Supplements can fill a specific gap, for example in some life stages, when food cannot. They are not automatically useful because they are sold beside a picture of a prostate.

What “evidence” should mean before you swallow a capsule

A strong study for a health claim usually assigns people by chance to the product or to a placebo, counts a health outcome that matters, and is large enough that a chance finding is less likely. Weaker evidence includes laboratory experiments, animal studies, and surveys that notice a link between a habit and a later diagnosis. Those weaker designs can suggest a question. They cannot show that a bottle will produce the same result in a man who is different from the people studied, or at a dose the study never used.

“Natural” does not mean safe. Selenium is natural and necessary, and too much of it causes hair loss, nail changes, stomach illness, and nerve injury. The adult upper limit from food and supplements together is 400 micrograms a day. Plants and herbs can interact with medicines. A product can also contain something other than the ingredient on the front label. Report a bad reaction to the FDA rather than assuming the next brand will be different only in its slogan.

Two ingredients men are often sold

Selenium and vitamin E. SELECT, a randomized trial of more than 35,000 men, did not show a preventive benefit against prostate cancer. Vitamin E alone, at 400 international units a day, was later linked with a statistically significant increase in prostate-cancer diagnoses. Men who already had high selenium levels and took selenium had a higher chance of high-grade cancer in a follow-up analysis. The National Cancer Institute says men should not take these supplements to prevent cancer. Food sources of selenium are a separate issue. Most people in the United States already meet their needs from meals.

Saw palmetto. It is promoted for urinary symptoms of an enlarged prostate, for chronic pelvic pain, and for hair loss. The National Center for Complementary and Integrative Health says there is enough evidence to conclude that saw palmetto is probably not helpful for urinary symptoms from enlargement. A 2023 review of 27 studies found little or no benefit when the herb was used alone. Two National Institutes of Health trials, one at a usual dose and one at rising doses up to three times that dose, did not improve symptoms. A 2022 review did not find a significant benefit for chronic prostatitis or chronic pelvic pain. The herb has been tolerated in studies for as long as three years, with mild digestive symptoms, dizziness, or headache in some people. Tolerance is not effectiveness. It does not appear to change PSA readings, which means it also will not “hide” a PSA result in a useful way, and it should not be used as a reason to skip an evaluation.

Tell a clinician what you take

Supplements can interact with prescription drugs. Cisplatin, a cancer medicine, can lower selenium levels, though the Office of Dietary Supplements says the effect of that change is unclear. Other interactions depend on the product. Bring the bottles, not a nickname from an advertisement. Stopping a prescribed medicine in favor of a supplement is not a step this site supports.