Got Cancer? Supplements May Be Dangerous

If you have cancer, there is a reasonable chance that you are taking supplements. Should you?

IF YOU HAVE CANCER, there is a reasonable chance that you are taking supplements. The use of dietary supplements in the United States is widespread. Many of my patients offer that their supplement use can lower their risk of cancer returning. But are they mistaken in their belief?

Here are the numbers — Among patients found to have breast, colorectal, or prostate cancer:

  • 40 percent reported dietary supplement use
  • 19 percent believed dietary supplements could reduce cancer recurrence risk

Researchers recently reported these findings in the journal Cancer.

There appeared to be differences in supplement use by subgroups. Females appeared nearly 2.5-times more likely to take supplements. Perhaps not surprisingly, a belief in the importance of supplements to reduce cancer recurrence risk led to a 3.1-times higher chance of supplement use. Obese individuals were nearly half as likely to use dietary supplements.

What are the most common supplements? Thirteen percent reported fish oil use, while 9 percent used calcium (with or without vitamin D). Over eight percent consumed multivitamins and minerals. For those with breast cancer, 15 percent used calcium (with or without vitamin D).

Do supplements improve cancer outcomes?

Study author Rana E. Conway, BSc (Hons), Ph.D., RNutr, research fellow in the obesity group at the research department of behavioral science and health at University College London, offers this:

“There is no evidence that self-prescribed supplements reduce the risks of cancer coming back, and they could interfere with treatment.”

There is good evidence that a healthy diet and physical activity are beneficial; supplements appear to be an easier option, but we don’t have evidence that they prevent cancer from coming back.

On the other hand, mixing supplements with chemotherapy comes with peril. While common, such supplement use can lead to serious potential medication interactions. A 2021 study demonstrates this problem.

Prescription medications are most often associated with drug interactions, followed by herb and supplement-related interactions. Over one-third of potential medication interactions are considered significant.

Moreover, supplements may lead to poorer outcomes for those receiving chemotherapy for cancer. In a study of patients with breast cancer, patients who took vitamin B12 before and during chemotherapy had poorer disease-free and overall survival odds.

Iron consumption before and during chemotherapy appeared associated with a higher probability of a return of cancer. On the other hand, multivitamin use did not seem linked to changes in survival.

Certain antioxidants may help you fight cancer (or provide protection for your normal cells). However, we know that some supplements may make cancer treatment less successful. If you have cancer management, please ensure that your care team knows about any supplements you are taking.

Photo by Michele Blackwell on Unsplash

Do supplements improve life length?

Moving beyond the realm of cancer, do supplements improve mortality? Do we live longer if we add this form of nutrients into our diets? While adequate intake of specific nutrients derived from food appears associated with lower all-cause mortality, nutrients derived from supplements are not helpful.

When Fang Fang Zhang, MD, Ph.D. (of Tufts University) evaluated data from the 1999 to 2010 National Health and Nutrition Examination Survey (NHANES) and National Death Index to determine how dietary supplement use and levels of nutrient intake from foods affect all-cause, cardiovascular disease, and cancer mortality among adults in the U.S. aged 20 years or older, she discovered:

  • There appeared to be no association between the ever-use of dietary supplements and mortality outcomes.
  • Participants with adequate vitamin A, vitamin K, magnesium, zinc, and copper via foods, not supplements, had lower all-cause and cardiovascular disease mortality rates.

Dr. Zhang offers this: “The general U.S. population should aim for achieving adequate nutrition through a healthy and balanced diet rather than counting on dietary supplements.

She adds: “For certain subgroups such as individuals with medical conditions that lead to malabsorption of nutrients from foods or those who have specific dietary practices that could cause nutritional deficiency, their nutritional needs including the use of dietary supplements shall be evaluated separately.”

These findings remind us that we health care professionals should review the use of dietary supplements with our patients. Thank you for joining me today.


Medika Life has provided this material for your information. It is not intended to substitute for the medical expertise and advice of your health care provider(s). We encourage you to discuss any decisions about treatment or care with your health care provider. The mention of any product, service, or therapy is not an endorsement by Medika Life

Michael Hunter, MD
Michael Hunter, MD
I received an undergraduate degree from Harvard, a medical degree from Yale, and trained in radiation oncology at the University of Pennsylvania. I practice radiation oncology in the Seattle area.
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