Dietary supplements tend to be sorely misunderstood. While a few may help manage certain health conditions, they are not medicines and aren’t meant to treat diseases. But many people take them, often by the handful, and believe they are beneficial to their health.

This can be especially true among people who have or have had cancer, new research finds. Many cancer patients and former cancer patients use dietary supplements because they mistakenly hope it will help treat their cancer or prevent its recurrence.

The American Cancer Society’s guidelines for cancer prevention include eating a healthy diet and engaging in physical activity. Dietary supplements are not mentioned.

Nutritional supplements do not prevent or cure diseases. In fact, they can have serious side effects and interfere with medical treatments. Some vitamins or minerals may even interfere with how well cancer drugs work.

Medicines need approval from the Food and Drug Administration (FDA) before they can be put on store shelves, but dietary supplements do not require such approval. While they don’t require a prescription, dietary supplements are also not required to meet the same requirements for safety and effectiveness that medicines must meet.

Researchers at University College London surveyed just over 1,000 adults enrolled in the Cancer Research UK-funded Advancing Survival Cancer Outcomes Trial who had been diagnosed with either breast, prostate or colorectal cancer. Each participant completed a survey that was mailed to them and a phone or online dietary analysis that included questions about their use of dietary supplements.

Some vitamins or minerals may even interfere with how well cancer drugs work.

Forty percent of cancer patients surveyed took dietary supplements, and 19 percent believed they would reduce the risk of their cancer coming back. Among the people most likely to use dietary supplements were women, those who ate five fruits and vegetables a day, and people who believed dietary supplements were vital to reduce the risk of cancer recurrence. Obese people were not as likely to use dietary supplements.

Fish oil was the most commonly used dietary supplement, followed by calcium with or without vitamin D, multivitamin/mineral supplement and vitamin D. Among people with breast cancer, 15 percent took calcium with or without vitamin D.

“As the number of people living with or beyond cancer increases, there is a growing need for a more holistic approach to long-term health care,” researcher, Rana Conway, of University College London, said in a statement. “Information about the role of supplements and the lack of evidence that they reduce cancer recurrence would be beneficial, alongside discussions about the benefits of healthy eating and physical activity.”

The American Cancer Society’s guidelines for cancer prevention include eating a healthy diet and engaging in physical activity. Dietary supplements are not mentioned. The findings of this study indicate that health care professionals should be advising cancer patients about the use of dietary supplements. A registered dietitian nutritionist who is trained to provide this kind of information would be a good choice for cancer patients hoping to protect their health.

A poor diet plus dietary supplements is still a poor diet. Supplements were never intended to take the place of a healthy diet. Food should be used to meet nutritional requirements and support good health for cancer patients and everyone else.

The study is published in Cancer.