No supplement has been proven to prevent cancer. That’s the short, honest answer. But the longer answer is more nuanced: a handful of supplements have shown modest effects on cancer mortality or precancerous growths in large clinical trials, while others once considered promising have turned out to be useless or even harmful. Here’s what the best available evidence actually shows.
Vitamin D: Lower Mortality, Not Fewer Cancers
Vitamin D is the closest thing to a supplement with meaningful cancer-related benefits, but the benefit isn’t what most people expect. A meta-analysis of randomized controlled trials found that vitamin D supplementation did not reduce the rate of new cancer diagnoses at all. What it did reduce was cancer death, by about 13%.
The largest trial behind this finding, called VITAL, gave participants 2,000 IU of vitamin D daily. When researchers excluded the first two years of follow-up (since supplements wouldn’t realistically affect cancers already developing), the reduction in cancer deaths grew to about 25%. With stricter confirmation of cause of death, it reached 37%. The most likely explanation is that vitamin D doesn’t stop cancer from forming but may slow its progression or improve the body’s ability to fight it once it starts. The benefit was driven by daily moderate doses rather than infrequent large doses.
Calcium and Colorectal Polyps
Calcium supplements have shown a consistent, modest ability to reduce the recurrence of colorectal adenomas, the precancerous polyps that can develop into colon cancer. A meta-analysis of high-quality randomized trials found a 12% reduction in adenoma recurrence with calcium supplementation over three to five years. When all trials were pooled, including smaller ones, the reduction was 17%.
Higher doses appeared more effective. Trials using at least 1,600 mg of elemental calcium daily saw a 26% reduction, compared to 16% with doses of 1,200 mg or less. This is relevant only for people who have already had polyps removed and are trying to prevent new ones. There’s no strong evidence that calcium prevents a first cancer diagnosis in people without a history of polyps.
Multivitamins Don’t Move the Needle
A daily multivitamin is one of the most common supplements people take “just in case.” The COSMOS trial, a large randomized study of older adults, tested this directly. Over a median follow-up of 3.6 years, 518 people in the multivitamin group developed invasive cancer compared with 535 in the placebo group. That difference was not statistically significant. A daily multivitamin does not reduce cancer risk.
Selenium and Vitamin E: A Cautionary Tale
For years, selenium and vitamin E were promoted as prostate cancer preventatives based on early observational data. The SELECT trial, one of the largest cancer prevention trials ever conducted, tested both supplements in nearly 30,000 men. Neither selenium nor vitamin E reduced prostate cancer risk. In fact, vitamin E supplementation was associated with a small but real increase: 17% more prostate cancers compared to placebo. Selenium showed a non-significant trend toward increased risk as well.
This trial is a key reason cancer researchers are cautious about supplements. Observational studies suggested benefit. A rigorous trial showed the opposite.
Beta-Carotene: Harmful for Smokers
Beta-carotene, the pigment that gives carrots their orange color, was once considered a promising cancer-prevention supplement because people who ate more fruits and vegetables rich in beta-carotene had lower cancer rates. Then the ATBC trial tested it in nearly 30,000 male smokers and found a 16% increase in lung cancer among those taking beta-carotene supplements. The trial was stopped early because participants in the supplement group were developing more cancer, not less.
This remains one of the starkest examples of why isolated supplements don’t replicate the effects of whole foods. The protective compounds in fruits and vegetables likely work together in ways that a single extracted ingredient cannot reproduce.
Folic Acid: A Double-Edged Sword
Folic acid illustrates how the same nutrient can behave very differently depending on context. At low doses (under 400 micrograms per day), population-level data from food fortification programs has been associated with declines in certain cancers, including childhood cancers. But at higher supplemental doses, folic acid may actually feed existing cancers.
Cancer cells divide rapidly and need large amounts of the building blocks for DNA. Folic acid supplies exactly those building blocks. A large trial gave 1 mg of folic acid daily to people who’d had colorectal polyps removed and found no protective effect. Instead, there was a notable increase in recurrent adenomas and a marginally significant rise in high-risk adenomas. Routine intake above 400 micrograms per day has also been linked to increased breast cancer risk. In one trial of pregnant women, those given 5 mg per day had a 70% greater risk of total cancer than the placebo group.
If you’re taking a prenatal vitamin or eating fortified foods, you’re likely getting an appropriate amount. High-dose folic acid supplements beyond what’s needed for pregnancy or a diagnosed deficiency carry real risk, particularly for anyone with precancerous growths.
Curcumin: Interesting but Preliminary
Curcumin, the active compound in turmeric, has generated enormous interest. In one small study, patients who took 4,000 mg of curcumin daily saw a 40% decrease in aberrant crypt foci, an early marker of colorectal cancer. Another trial found that a curcumin-containing product produced more clinical responses in oral precancerous lesions than placebo. These results are genuinely intriguing, but the studies are small and the doses are far higher than what you’d get from cooking with turmeric. Curcumin is also poorly absorbed, which is why supplement makers often pair it with other compounds to boost uptake. It’s too early to recommend curcumin supplements for cancer prevention based on this evidence.
What Actually Works
The American Cancer Society’s guidelines for cancer prevention focus almost entirely on food and lifestyle, not pills. The ACS recommends getting nutrients through whole plant foods rather than supplements. Even fiber supplements, which might seem like a logical substitute for a high-fiber diet, have not been shown to reduce colon polyps in trials.
The pattern across decades of research is consistent: nutrients in food seem to protect against cancer. The same nutrients extracted into pills generally do not, and sometimes cause harm. This likely reflects the fact that whole foods contain thousands of compounds working in concert, buffering and amplifying each other in ways that a single isolated ingredient cannot.
If your vitamin D levels are low, supplementation makes sense for many health reasons, and the mortality data is encouraging. Calcium may benefit people with a history of colorectal polyps, ideally discussed with a gastroenterologist. Beyond those specific situations, the most evidence-backed cancer prevention strategy remains unglamorous: eat more vegetables, fruits, and whole grains, maintain a healthy weight, stay physically active, limit alcohol, and avoid tobacco.