WHILE DIETARY SUPPLEMENTS OR PRODUCTS containing vitamins, minerals or other nutrients can be useful in some cases—for example, iron and folic acid supplements for people who are or plan to become pregnant—research has not supported them as a tool for preventing cancer.

Supplements for cancer prevention “do not work,” says Nigel Brockton, vice president of research at the American Institute for Cancer Research (AICR). “And there’s also evidence that they may actually be harmful.”

Going back decades in some cases, researchers have tested if supplements prevent cancer. “These trials have been done because people thought they [supplements] would be helpful,” Brockton says. “And even in that rigorously tested setting, the outcomes are actually the opposite of what people expected. And they’re associated with poorer outcomes.”

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According to a recent report by the World Cancer Research Fund and the AICR, clinical trials have not established a consistent effect of dietary supplements on cancer prevention. For example, a study of beta-carotene for cancer prevention in heavy smokers was stopped early because participants in the supplement group had a higher incidence of lung cancer. Another study testing use of vitamin E to prevent secondary cancers in people with head and neck cancer found that those taking the supplement experienced higher rates of recurrence, higher incidence of secondary cancers and lower cancer-free survival rates. 

Supplements may also reduce the efficacy of cancer treatments. A 2019 study in Germany found that breast cancer patients who used antioxidant supplements during chemotherapy and radiation had higher risks of cancer progression and recurrence and were more likely to die of any cause.

Talk to Your Doctor

Some supplements may interfere with cancer treatments.

For these reasons, Brockton says, the AICR does not recommend supplements for cancer prevention. “If you eat a plant-based diet that has higher levels of these micronutrients [also contained in supplements], you have a reduced risk of cancer,” Brockton says. “But when you isolate those antioxidants [and] phytochemicals and try to use those as cancer fighting, they’re not effective,” he says.

Ashley P. Taylor is a writer and science journalist based in New York City.