Using Supplements During Chemo: Yes or No?

Posted on 8/31/2022 by Karen Sabbath, MS, RD, CSO

Americans spend more than $30 billion a year on vitamins, herbals and other supplements. It is not uncommon for our patients to arrive to their oncology consultations with a large bag (or long list) of the supplements they are taking. It has been estimated that 65%-80% of all cancer patients take one or more vitamin or mineral supplements, and many of these patients START taking supplements just prior to starting their treatment.

The problem is that many supplements actually have the potential to interfere with the cancer treatment, in some cases making the treatment less effective. Plus, supplements can interact with other medications, resulting in delivering too high or too low of a dose or triggering unwanted side effects.

Keep in mind, there are some supplements that are prescribed due to pre-existing deficiencies or deficiencies caused from treatment, and these are medically necessary. Some examples would be correcting a chemotherapy-induced magnesium deficiency, or replenishing someone’s vitamin D or vitamin B-12 levels if they are low.

Why is supplement use during cancer treatment, particularly of antioxidants, a problem?

Many studies have indicated that taking antioxidant supplements (foods are fine) during chemotherapy and/or radiation treatment was associated with a lower breast cancer survival rate and may hasten the spread of lung cancer, as well as some other cancers. One study observed that taking vitamin C with Tamoxifen decreased its effectiveness.

How does this work?

It’s all about “free radicals,” which are highly charged oxygen complexes in your body that can be created by cigarette smoke, exposure to radiation, environmental toxins, or an unhealthy diet. Too many free radicals can result in DNA changes and cell damage that may lead to diseases including cancer. Antioxidants, which can be produced by our bodies or obtained from a diet rich in fruits and vegetables, can neutralize these free radicals and prevent cell damage. Getting your antioxidants from food is the best way to reap the benefits. Getting them from supplements does not appear to provide the same benefits.

So, what’s the problem?

Many cancer treatments work by creating free radicals that then go on to destroy cancer cells. Taking antioxidants in supplement form (again, remember that antioxidants in food are fine) may actually “protect” cancer cells during treatment. In other words, antioxidants in pill form have the potential to counteract the effects of chemotherapy or radiation therapy.

Common antioxidant supplements taken by patients include vitamins A, C, and E, carotenoids (such as beta-carotene and lycopene) as well as selenium and Coenzyme Q10. We generally recommend that patients discontinue them before, during and for a period of time after treatment is completed.

Other supplements can increase or decrease blood sugar, blood pressure, or bleeding time. If you are taking medications for any of these medical conditions, make sure that someone reviews what supplements you are taking to be certain that there will be no adverse effects to your health.

Are there any exceptions?

Vitamin D, which is NOT an antioxidant, is often called “the sunshine vitamin” and deserves special consideration. Because it is not found in foods, it can be difficult to obtain enough naturally through sun exposure. It is easy to get a blood test to determine your vitamin D level. Low levels of vitamin D have been associated with increased risk of certain cancers, and higher levels of vitamin D are associated with a lower risk of developing breast and colon cancer and improved survival from lung and colon cancer. It is important not to overdo vitamin D supplementation, since blood levels which are too high can increase inflammation. Your doctor can help you determine the best way to increase your levels if they are low.