Fish Oil, Herbs and Supplements to Avoid with Breast Cancer Treatment

Patient Expert

Researchers in the Netherlands found that healthy volunteers who were given fish oil or fatty fish like salmon and mackerel showed a significant increase in their blood levels of a fatty acid that has been associated with resistance to chemotherapy in earlier studies. The study at the University Center Utrecht concluded that patients on chemotherapy should avoid fish oil at least a day before and after their chemo treatments.

This was a small study, but it points out an issue that many cancer patients need to understand. Supplements or herbs that may be fine for healthy people may not be appropriate while in cancer treatment. For example, anti-oxidant vitamins can help cancer cells avoid the damaging effects of radiation. The whole point of going through radiation is to kill cancer cells, so you don’t want to do anything to help them survive.

My oncologist warned me not to take any supplements that had plant estrogens because estrogen can feed some breast cancers. Although my tumor was officially estrogen-receptor negative, she still felt that estrogen would not be my friend, especially in forms that my own body was not producing. Many breast cancer patients who are experiencing menopausal symptoms will have well-meaning friends who suggest “safe, natural remedies” for hot flashes or other problems related to low estrogen. If there is enough estrogen in an herb to make you feel better, there may be enough estrogen to help your cancer grow.

Avoiding herbs

Another group of supplements and herbs that may cause problems are the blood thinners like Vitamin E and fish oil. Many people take these hoping to prevent blood clots, but if surgery is going to be part of your treatment plan, you will need to stop these before your operation.

Dr. June McKoy of the Lurie Comprehensive Cancer Center at Northwestern University writes, “Some herbs can interfere with the metabolism of the [chemotherapy] drugs, making them less effective while other herbs such as long-term use of garlic may increase the risk of bleeding during surgery. While culinary herbs used in small quantities for flavoring are generally safe, consuming large amounts for prolonged periods of time may have a negative effect on the body when going through chemotherapy."

For 17 years I have participated in on-line support groups for cancer patients, and I have noticed that one of the first questions many new patients ask is, “What supplements or natural remedies should I take to help my body fight cancer?” The problem is that an herb strong enough to help your healthy cells grow and survive may be making your cancer cells stronger.

The last time I went in for a surgery, the pre-op interview asked me many questions about supplements and had a specific list of ones to stop taking a week before the operation. Doctors are more aware these days of the interest patients have in supplement use. You may find your doctor more receptive than you expect to recommending appropriate herbs or supplements for your situation. For example, if your Vitamin D levels are low, your doctor may recommend taking Vitamin D, which is associated with good breast health.

Breast cancer is not a one-size-fits-all disease, so an herb that your friend’s doctor says is fine for her may not be good for you. It is important to let your doctors know about every single vitamin, herb, or supplement you are considering. Your doctor can help you assess the risks and benefits for you.

Read More About It:

Supplements: Do They Help or Hurt?

Does Vitamin D Help Breast Cancer Patients?

An Introduction to Chemotherapy

Radiation 101: Your Guide to Radiation

Vitamins to Avoid During Radiation


Azvolinsky, A. “Fish Oil Consumptions Linked to Chemoresistance.” Cancer Network. April 9, 2015. Accessed from June 11, 2015.

McKoy, J. “Herbal Supplements May Affect Chemotherapy.” Robert H. Lurie Comprehensive Cancer Center at Northwestern University. July 2011. Accessed from June 11, 2015.