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Study: Can taking dietary supplements during chemotherapy do more harm than good?

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What does this mean for me? In-depth            
Clinical trials Limitations
Guidelines Resources


This study is about:

Whether using vitamins and dietary supplements before and during chemotherapy can affect treatment results. 

Why is this study important?

The use of dietary supplements is common including after a cancer diagnosis. However, taking dietary supplements before and during chemotherapy may reduce the ability of chemotherapy to kill cancer cells. This study looks at how the use of dietary supplements before or during chemotherapy affects treatment outcomes.

Most of the media outlets reporting on this study incorrectly stated that any antioxidant use was shown to increase the risk of breast cancer recurrence and death. However, these results were not statistically significant and call into question the reporting. See the limitations section for more details.

Study findings: 

The study enrolled male and female breast cancer patients diagnosed at stages 1-3. Among participants undergoing chemotherapy (doxorubicin, cyclophosphamide and paclitaxel):

  • Use of vitamin B12 both before and during chemotherapy was related to poorer disease-free survival and overall survival.
  • Use of iron during chemotherapy was related to recurrence.
    • Use of iron both before and during treatment was related to increased recurrence. Increased recurrence was higher for patients who used iron only during chemotherapy.
    • Multivitamin use was not linked to changes in survival outcomes.

What does this mean for me?

Use of vitamin B12 and iron during chemotherapy was linked to poorer outcomes. However, while results of this study hint of a connection between use of antioxidants and outcomes, a larger study is needed to understand whether these popular nutritional supplements definitely affect chemotherapy outcomes for cancer patients.

The results of this study support current recommendations that patients use caution and speak with their oncologist before taking supplements during chemotherapy. 

Posted 3/4/20.

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Ambrosone CB, Zirpoli GR, Hutson AD, McCann WE, McCann SE, Barlow WE, Kelly KM, Cannioto R, Sucheston-Campbell LE, Hershman DL, Unger JM, Moore HCF, Stewart JA, Isaacs C, Hobday TJ, Salim M, Hortobagyi GN, Gralow JR, Budd GT and Albain KS. “Dietary Supplement Use During Chemotherapy and Survival Outcomes of Patients With Breast Cancer Enrolled in a Cooperative Group Clinical Trial (SWOG S0221)”. J Clin Oncol. 2019 Dec 19.


FORCE receives funding from industry sponsors, including companies that manufacture cancer drugs, tests and devices. All XRAYS articles are written independently of any sponsor and are reviewed by members of our Scientific Advisory Board prior to publication to assure scientific integrity.

This article is relevant for:

People undergoing breast cancer treatment with chemotherapy

This article is also relevant for:

people with breast cancer

people with ER/PR + cancer

people with Her2-positive cancer

men with breast cancer

people with ovarian cancer

people with triple negative breast cancer

people newly diagnosed with cancer

people with pancreatic cancer

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Study background:

Although most people diagnosed with breast cancer never have a recurrence, it can return at any time. In general, breast cancer recurs in about 30 percent of patients, but everyone who has had the disease is at risk of recurrence. Most recurrences occur in the first three to five years after initial treatment.

Following a cancer diagnosis, patients may be motivated to make lifestyle changes that they believe will improve their health, reduce their risk of recurrence and reduce treatment side effects. Patients often take dietary supplements to complement their traditional cancer therapies. Reports of patients’ use of dietary supplements after diagnosis range from 60 percent to more than 80 percent. However, dietary supplements may reduce the effectiveness of chemotherapy. The American Cancer Society states that, “So far, no dietary supplements (including vitamins, minerals, and herbal products) have been shown to clearly help lower the risk of breast cancer progressing or coming back.” Indeed, most patients are advised to speak to a member of their healthcare team if they are considering taking supplements.

Researchers of this study wanted to know:

Whether an association exists between the use of dietary supplements and breast cancer treatment outcomes.

Populations looked at in this study:

SWG S0221 is a clinical trial that compared different treatment schedules in female and male patients with breast cancer. All partipants had breast cancer ( I-III). The Dietary, Exercise and Lifestyle and Cancer Prognosis (DELCaP) study enrolled patients who participated in SWG S0221 to understand how their dietary patterns, exercise and other lifestyle factors might affect their cancer treatment outcomes.

Study design:

Of eligible patients enrolled in SWOG S0221, 1,607 agreed to participate in DELCaP. Before beginning chemotherapy, patients completed a baseline questionnaire about their regular use of dietary supplements before diagnosis and between diagnosis and enrollment in the trial. Regular use was defined as using a supplement at least once a week. When chemotherapy was scheduled to be completed (about six months after randomization to treatment schedule), patients were asked to complete a second questionnaire about their supplement use during their chemotherapy.

Disease-free survival was defined as the period from the end of chemotherapy (among living patients without recurrence) until the first documentation of breast cancer recurrence, diagnosis of a new breast cancer or death.

Overall survival was defined as the period from the end of chemotherapy until death from any cause. Median follow-up time was 8.1 years.

Study findings:  

Among the 1,134 patients who completed both questionnaires, 251 had a breast cancer recurrence and 181 died. The frequency of supplement use, specifically antioxidant use, among these patients was low compared to reports in the literature. Use of supplements tended to decease during treatment. For example, vitamin C was used by 20 percent of patients before treatment but by only 12 percent during therapy. Vitamins E and A were taken by fewer than 10 percent of patients during treatment. About 17 percent of patients used any antioxidant (vitamins C, A and E; carotenoids; or coenzyme Q10) during treatment. Multivitamins were used by 44 percent of patients during chemotherapy.

  • Multivitamin use was not associated with survival outcomes.

For supplements other than antioxidants:

  • Use of vitamin B12 both before and during chemotherapy was associated with poorer disease-free survival and poorer overall survival.
  • Use of iron during chemotherapy was associated with recurrence, as was use both before and during treatment.
    • Results were similar for overall survival.

For antioxidants:

  • It is unknown if use of any antioxidant both before and during treatment effects chemotherapy outcomes.
    • No correlation with outcomes was made for use of antioxidants only before or during therapy.
    • No associations were observed between use of single antioxidants and outcomes.

The authors state, “There were indications that use of any antioxidant (vitamins A, C, and E; carotenoids; coenzyme Q10) both before and during treatment was correlated with an increased risk of recurrence and to a lesser extent death.” While researchers did observe that patients who took any antioxidants had up to a 41 percent increased risk in recurrence and a 40 percent increased risk in death, these results are not statistically significant. Despite the authors claim that antioxidants are correlated with increased risks, their data does not support this. It is unclear whether this correlation is real, and a larger study with more people would be needed to prove this link if there truly is one or not.


The strength of this study was that all breast cancer patients received the same chemotherapy, although their treatment schedules were different. Despite this strength, as with any observational study, there are limitations including selection bias, imperfect recall and other confounding factors. For example, it has been reported that up to 60 percent of breast cancer survivors began using dietary supplements following a diagnosis. However, the use of dietary supplements in this study was much lower, although it is unclear why.

The study did not quantify supplement dose, frequency, or brand.  This limitation is compounded by the small sample size. It is also unknown how much of the needed micronutrients and minerals people are obtaining through their diet. Patients undergoing chemotherapy may experience significant dietary changes (loss of appetite) that are not addressed in this study. Additionally, patients may have underreported use or they may have adhered more closely to recommendations from their healthcare providers to avoid supplements during treatment.

The small number of patients taking dietary supplements in this study limits its ability to identify correlations and may lead to false results.


The results of DELCaP showed an association between some supplements (B12 and iron) and survival outcomes for (I-III) breast cancer patients who were undergoing chemotherapy. However, these results support the current clinical recommendation that patients use caution when considering the use of dietary supplements other than a multivitamin during chemotherapy.

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Expert Guidelines
Expert Guidelines

Dietary Supplements

National Comprehensive Cancer Network (NCCN) guidelines on survivorship include the following recommendations on dietary supplement use:

  • Taking dietary supplements is not recommended for most cancer survivors unless a patient has a known nutritional deficit, an inadequate diet or other indication (for example, ).
  • Little data exist to support the use of vitamins or other dietary supplements for cancer prevention, control or recurrence.
  • Taking vitamin supplements does not replace the need for a healthy diet. Patients should try to get nutrients from the foods they eat and the beverages they drink.
  • Providers should ask about supplement use at regular intervals, about a patient’s reasons for using supplements and the ingredients in those supplements.
  • Survivors of certain cancers are at risk of vitamin deficiencies based on cancer treatment (e.g., gastric cancer patients who have had a gastrectomy may be at risk of vitamin B12 and iron deficiencies).
  • NCCN recommends calcium and vitamin D supplements for people who have been prescribed denosumab or a bisphosphonate to treat bone or .
  • Patients taking multiple supplements and those in need of nutritional support should be referred to a registered dietitian or nutritionist, preferably one who is trained in supporting oncology patients. 

Updated: 05/20/2022

Expert Guidelines
Expert Guidelines

Nutrition for people diagnosed with cancer

The National Comprehensive Cancer Network (NCCN) recommends the following for cancer survivors: 

  • Think about your food choices and amount of fruits, vegetables and unrefined grains you eat compared with red and processed meats, alcohol, and foods or drinks with added fat or sugar. 
  • Think about your eating habits, including portion size, snacking, how often you eat out and use of added fats or sugars.
  • All survivors should be encouraged to:
    • Make informed choices about food to ensure variety and adequate nutrient intake.
    • Limit refined sugars.
    • Eat a diet that is at least 50% plant-based, consisting mostly of vegetables, fruit and whole grains.
    • Track calorie intake; monitoring of calories is an effective way to manage weight.
    • Minimize alcohol intake to no more than one drink per day for a woman and two drinks per day for a man.
  • For patients desiring further dietary guidelines, the USDA approximate food plate volumes are:
    • Vegetables and fruits should comprise half the volume of food on the plate
    • Vegetables: 30% of plate; Fruits 20% of plate
    • Whole grains: 30% of plate
    • Protein: 20% of plate
  • Recommended sources of nutrients:
    • Fat: plant sources such as olive or canola oil, avocados, seeds and nuts, and fatty fish.
    • Carbohydrates: fruits, vegetables, whole grains and legumes.
    • Protein: poultry, fish, legumes, low-fat dairy foods, and nuts.

The Academy of Nutrition and Dietetics, the United States Office of Disease Prevention and Health Promotion  and the American Institute for Cancer Research also publish expert guidelines on nutrition and health. 

Updated: 12/12/2021

Questions To Ask Your Doctor
Questions To Ask Your Doctor

  • Should I consider taking vitamins or other dietary supplements before or during chemotherapy?
  • Should I consider taking vitamins after finishing chemotherapy?
  • My taste, appetite, or ability to eat have changed due to side effects from my treatment. How can I make sure that I am getting all the nutrients?
  • How do I know if my diet is balanced?
  • Can you refer me to a nutritionist?

Open Clinical Trials
Open Clinical Trials

The following are studies focused on nutrition for people diagnosed with breast cancer. 

Visit our Featured Research Page and Research Search and Enroll Tool to find additional studies enrolling people with, or at high risk for cancer.

Updated: 05/29/2023

Find Experts
Find Experts

The following resources can help you locate a nutritionist near you or via telehealth

Finding nutritionists

  • You can find a registered dietician in your area through, the website for the Academy of Nutrition and Dietetics. Search for nutritionists by specialty, including "cancer," "weight management" and "heart health."

Related experts

  • The YMCA has a free program called Livestrong at the YMCA. This program includes a free 12-week membership and fitness training with certified exercise experts. You can search by zip code for a program near you.  

Other ways to find experts

  • Register for the FORCE Message Boards and post on the Find a Specialist board to connect with other people who share your situation. 
  • The National Cancer Institute (NCI)-designated comprehensive cancer centers have specialists to manage the symptoms and side effects from cancer prevention or treatment. 
  • FORCE partners with Savor Health® to provide free, personalized, evidence-based nutrition support 24/7 and “on-demand" through their text-based Intelligent Nutrition Assistant (Ina®). You can subscribe here


Updated: 11/20/2023

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