Meg Seymour, PhD, National Center for Health Research
Many Americans, including those with cancer, take dietary supplements. People take supplements because they believe it will help them stay healthy and give them vitamins and minerals they may not get from their diet. Chemotherapy patients often take supplements because their nausea makes it difficult to eat, and they want to be sure to get enough nutrients.
People think of dietary supplements as a no-risk insurance policy to improve nutrition, but a study published in 2020 shows that supplements can have risks if you are undergoing chemotherapy. More than 1,000 breast cancer patients were asked whether or not they took any supplements either before or during their chemotherapy.1 The researchers then continued to evaluate any subsequent cancer or death for up to 15 years (almost all of the women were followed for at least 5 years).
Results showed that patients who took vitamin B12 both before and during their chemotherapy were more likely to develop cancer again in the 5-15 years after treatment and were more likely to die as a result. They were also more likely to die from any cause, not just from cancer. This increase in subsequent cancer or death was only for people who took the supplements both before and during their chemotherapy. Patients who only took the supplements before chemotherapy or only took supplements during chemotherapy were not more likely to develop cancer again or to die from cancer or other causes in the years after treatment. Patients who took iron supplements both before and during chemotherapy were also more likely to develop cancer again after treatment or to die of cancer or any cause. However, the same was also true for people who only took iron supplements during their chemotherapy.
Results showed that patients who took vitamin B12 before and during their chemotherapy were more likely to die or have their cancer return. They were also more likely to die from any cause, not just from cancer. This increase in cancer recurrence or death was only for people who took the B12 supplements both before and during their chemotherapy. Patients who only took the B12 supplements before chemotherapy or only took supplements during chemotherapy were not more likely to have a recurrence of their cancer or die. Patients who took Iron supplements both before and during chemotherapy were also more likely to have their cancer return or to die of any cause. However, the same was also true for people who only took Iron supplements during their chemotherapy.
The researchers also looked at antioxidant supplements, which include vitamins A, C, and E. They found that patients taking these supplements both before and during chemotherapy had a 41% higher chance of cancer returning after treatment. However, this finding was not “statistically significant,” which means that more research is needed to determine whether these worse outcomes occurred by chance. The 44% of the patients in the study who were taking multivitamins did not have better or worse outcomes than people who were not taking them.
This is what scientists call an observational study rather than a clinical trial. In a clinical trial, some patients would be randomly assigned to take supplements and others would be assigned to take a placebo (with no active ingredients). In an observational study, people make their own decisions about what treatment (in this case supplements) to take. Those who chose to take supplements might have different health issues or health habits than those who did not. For example, it is possible that the people who were more likely to take supplements both before and during their chemotherapy were less healthy to begin with. For example, they could have been taking B12 or Iron supplements because they had anemia, and anemia may have increased the possibility of cancer recurrence or death. Also, because patients were asked whether or not they took supplements (instead of being given the supplements by researchers), it is impossible to know whether what patients said about supplements was completely accurate. For example, some patients could have said that they were regularly taking a supplement, but really they only took it occasionally.
Dr. Christine Ambrosone, the lead researcher of the study, said in an interview that this is only one observational study, and doctors should not necessarily base their recommendations on this single study. Doctors need to consider the specific needs of each patient. For example, someone with anemia might need a dietary supplement, and the benefits of those supplements might outweigh the potential risks.
If you are considering taking a dietary supplement, it is important to keep in mind that the Food and Drug Administration does not regulate dietary supplements for purity and quality. There is no guarantee that a supplement will work or even that it contains exactly what the bottle says it contains.2 It is always important to talk with your doctor to help you decide if the benefits of any dietary supplement you are considering outweigh the potential risks.
- Ambrosone, C. B., Zirpoli, G. R., Hutson, A. D., McCann, W. E., McCann, S. E., Barlow, W. E., … & Unger, J. M. (2019). Dietary Supplement Use During Chemotherapy and Survival Outcomes of Patients With Breast Cancer Enrolled in a Cooperative Group Clinical Trial (SWOG S0221). Journal of Clinical Oncology, JCO-19.
- Brooks, J, Mitchell, J., Nagelin-Anderson, E. , & Zuckerman, D. National Center for Health Research. How Safe are Natural Supplements? Center4research.org. http://www.center4research.org/examining-safety-natural-supplements/. 2019.