Healthy Eating Guidelines For After A Cancer Diagnosis: Dietary Supplements


Dietary supplements include vitamin and mineral supplements and natural health products such as herbs and other preparations. These are taken in addition to the foods you eat. People who have been diagnosed with cancer often wonder if dietary supplements will help to manage symptoms, fight cancer, strengthen the immune system, improve energy and ensure good nutrition.

Many people think dietary supplements are harmless because they are "natural," but after a cancer diagnosis dietary supplements can cause unwanted and sometimes harmful health effects. Dietary supplements may interfere with cancer treatments or other medications and may not be safe for people with specific kinds of cancers.

It is important to weigh the possible heath benefits and risks of taking dietary supplements, but because there is not very much research on the effects of many supplements for people diagnosed with cancer this can be challenging.

If you are having problems eating or have had unplanned weight loss this information may not be appropriate for you. Speak with a Registered Dietitian about your nutritional needs.

Steps You Can Take

Always speak with your doctor, pharmacist, nurse, or dietitian about whether it is safe for you to use dietary supplements during or after your cancer treatment. This is important because:

  • Dietary supplements might affect how cancer treatments work.
    • St. John's Wort, an herbal product, makes some chemotherapy drugs less effective.
    • Beta-carotene, vitamin C, vitamin E, and selenium, and other dietary supplements may act as antioxidants. Antioxidant supplements may affect how cancer treatments work and are not recommended if you are receiving chemotherapy or radiation therapy. Foods rich in antioxidants are safe to eat during cancer treatment as part of varied diet based on "Canada's Food Guide". A one-a-day multivitamin and mineral supplement that contains small amounts of a wide variety of nutrients including antioxidant nutrients is also acceptable.
  • Dietary supplements may not always be safe after a cancer diagnosis.
    • Some dietary supplements, such as DHEA and Siberian ginseng, contain male hormones and may increase testosterone levels. Male hormones may cause prostate cancer growth, so dietary supplements containing these are not recommended for men who have been diagnosed with prostate cancer.
    • Other dietary supplements, such as black cohosh and evening primrose oil, contain plant estrogens and may not be safe for women after a breast cancer diagnosis.
  • Large doses of vitamin, mineral, herbal or other supplements may cause unwanted health effects.
    • High amounts of vitamin E (more than 400 IU daily) or supplements of garlic, ginger, ginkgo and ginseng may increase bleeding risk.
    • Beta-carotene supplements increase the risk for lung cancer in smokers.

Use dietary supplements when appropriate.

  • A one-a-day multivitamin and mineral supplement may help improve your nutrition if you can't eat a well balanced diet during or after your cancer treatment. Look for a regular strength one-aday multivitamin and mineral supplement. This provides vitamins and minerals in amounts close to the daily recommendations for adults.
  • At different stages in life, if you have certain medical conditions, or if your food choices are limited, you may need to take specific vitamin and/or mineral supplements or fortified foods. For example:
    • People older than 50 years should take a 400 IU vitamin D supplement daily.
    • Men and women receiving certain hormone therapies for prostate and breast cancer may require additional calcium and vitamin D, especially if they are at risk for bone loss.
  • If you think dietary supplements would be helpful for you, talk with your dietitian, pharmacist, nurse or doctor.

Additional Resources

Dietitian Services Fact Sheets available by mail (call 8-1-1) or at

HealthLinkBC File #68k Vitamin and Mineral Supplements for Adults

BC Cancer Agency. Visit and select "Unconventional Therapies".

Natural Medicines Comprehensive Database. This resource contains information, including the product effectiveness and potential drug interactions, for thousands of natural products. You may be able to find the printed version in some libraries or you can purchase a monthly or yearly subscription to the website:

Memorial Sloan Kettering. This is a free searchable database about herbs, botanicals and other products and includes information on the common uses, how the therapy works, warnings and herb-drug interactions for each product. Visit

Last updated: October 2011

These resources are provided as sources of additional information believed to be reliable and accurate at the time of publication and should not be considered an endorsement of any information, service, product or company.

Distributed by:

Dietitian Services at HealthLinkBC (formerly Dial-A-Dietitian), providing free nutrition information and resources for BC residents and health professionals. Go to Healthy Eating or call 8-1-1 (anywhere in BC). Interpreters are available in over 130 languages.

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