Roughly 75% of adults in the United States use supplements.

CRN Survey Shows Consistent Supplement Usage with Increase of Specialty Product Use Over Time. Council for Responsible Nutrition. October 29, 2024.

But are they useful in cancer care? Oncologists say the answer isn’t so simple.

"Nutritional supplements are only necessary and recommended when a patient has a known nutritional deficiency," says Monica El-Masry, MD, a haematologist and oncologist at USC Norris Comprehensive Cancer Centre. "Supplements may interfere with the metabolisation of cancer drugs, leading to harmful side effects and potentially diminishing their efficacy."

The American Cancer Society and National Cancer Institute advise against the use of supplements while receiving treatment for cancer, such as chemotherapy and radiation therapy.

Are Dietary Supplements Safe? American Cancer Society. September 6, 2023.

This is primarily because supplements may increase the risk of kidney and liver toxicity.

Supplements can be dangerous during chemotherapy

While supplement usage is common in the US some supplements can be dangerous for cancer patients, Dr. El-Masry notes.

“A study from the Journal of Clinical Oncology, published in 2019, specifically studied the use of dietary supplements use during chemotherapy in patients with breast cancer. Use of any antioxidant supplement[s], including vitamin A, C, and E, carotenoids, coenzyme Q10, both before and during treatment, was associated with an increased risk of cancer recurrence and, to a lesser extent, death,” Dr. El-Masry says.

Ambrosone CB, Zirpoli GR, Hutson AD, et al. 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. 2020;38(8):804–814.

For non-antioxidants such as vitamin B12, use both before and during chemotherapy was associated with poorer disease-free survival and overall survival. Use of iron during chemotherapy was also significantly associated with recurrence.

In oncology, supplements should only be used in patients with a nutrient deficiency, as confirmed by a blood test, Dr. El-Masry says. She also advocates that maintaining a healthy and balanced diet is the best way to receive all essential vitamins and minerals.

The role of vitamin D

For some patients, however, vitamin D can play a role in both the risk of developing cancer and survival rates, Dr. El-Masry notes.

There is no supplement at this time that can cure cancer or keep it from coming back.

Some studies have shown a correlation between vitamin D deficiency and increased risk of developing colorectal cancer.

One study even reported an improved overall and progression-free survival with higher plasma vitamin D levels in patients with advanced or metastatic colorectal cancer, according to Dr. El-Masry.

Yuan C, Sato K, Hollis BW, et al. Plasma 25-Hydroxyvitamin D Levels and Survival in Patients with Advanced or Metastatic Colorectal Cancer: Findings from CALGB/SWOG 80405 (Alliance). Clinical Cancer Research. 2019 Dec 15;25(24):7497-7505.

“We know from studies that vitamin D is also very important for proper T-cell function, and it is very important that vitamin D levels are normal while receiving chemotherapy. Many chemotherapy drugs have severe side effects,” Dr. El-Masry says.