Do vitamins and supplements prevent cancer?
No. The National Cancer Institute's heading is blunt. Vitamin and dietary supplements have not been shown to prevent cancer. There is not enough proof that multivitamins, mineral supplements, or single vitamins can prevent it.
Eight have been studied and none has been shown to lower cancer risk: vitamins B6, B12, E, C, and D, plus beta carotene, folic acid, and selenium.
The trial that went the wrong way
In one large study, a supplement did the opposite of what was hoped.
The Selenium and Vitamin E Cancer Prevention Trial, known as SELECT, found that vitamin E taken alone raised the risk of prostate cancer. NCI adds a detail that matters. The risk continued even after the men stopped taking it. Taking selenium with vitamin E did not raise that risk. Nor did selenium alone.
That is why "it is only a vitamin" is not a safety argument. A widely sold supplement was taken by healthy men hoping to prevent cancer. It raised the risk of the very disease it was meant to prevent.
What the numbers on vitamin D actually say
Vitamin D is the one people ask about most, and NCI's answer is precise rather than dismissive.
Doses of 400 to 1,100 IU a day have not been shown to lower or raise the risk of cancer. A larger study is under way. The VITamin D and OmegA-3 TriaL, or VITAL, is testing 2,000 IU a day plus omega-3 fatty acids from oily fish.
There is one narrow positive finding, and it comes with limits. The Physicians' Health Study looked at men who had already had cancer. Those taking a daily multivitamin may have a slightly lower risk of a second cancer. That is one study, one group, and a small effect.
Aspirin gets similar treatment. NCI says the studies show mixed results. Most have shown that aspirin does not prevent cancer. There is evidence that long-term use may prevent colorectal cancer in certain people. But one randomized trial suggests aspirin may make cancer grow faster in the elderly, and longer follow-up is needed. Bleeding in the gut or brain is a known side effect.
What has been shown to work
NCI does not leave the shelf empty. Its list of what is known to lower risk names real interventions, and none of them is a supplement.
Drugs called selective estrogen receptor modulators, such as tamoxifen and raloxifene, cut breast cancer risk in women at high risk. Side effects such as hot flashes limit their use. Finasteride lowers prostate cancer risk. Weight loss surgery lowers the risk of cancers linked to excess weight. Surgery to remove the breasts, tubes, or ovaries is an option for some people with BRCA1 or BRCA2 changes. NCI says a cancer risk assessment and counseling come first.
Then there are the exposures NCI names as known causes. Cigarette smoking, which it says causes about 30% of all cancer deaths in the United States. Certain infections. And radiation, including ultraviolet light from the sun, the main cause of nonmelanoma skin cancers.
Two approved vaccines prevent cancer-causing infections. One prevents hepatitis B, which raises liver cancer risk. The other protects against strains of HPV that cause cervical cancer. Our guide to can supplements prevent cancer checks the claim in more detail, and cancer risk factors covers the best-supported ones.
One distinction, and one instruction
This answer is about taking supplements in the hope of preventing cancer. It is not about one prescribed to correct a deficiency found on a blood test. Nor is it about one needed after certain operations. Those are separate medical decisions.
For anyone in cancer treatment, NCI's instruction elsewhere is direct. Always check with a doctor before starting a supplement or alternative therapy. Some make cancer treatments work less well. Bringing the actual bottles to a visit works better than trying to recall doses.
NCI's prevention summary was last updated on May 2, 2025.
Sources
Want the full picture? Read our complete explanation: Cancer Prevention: An Overview
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