Vitamin and Mineral Supplements During Cancer Treatment

A question we are often asked is: Should I take vitamins during cancer treatment? It is an extremely important question considering a few statistics. In 2008, it was noted in the Journal of Clinical Oncology that between 64 and 81 percent of cancer patients were using a vitamin or mineral supplement (in contrast to 50 percent of the general population) and that between 14 and 32 percent of these people started taking supplements after their diagnosis of cancer. This is even more concerning now that studies in 2019 found that antioxidant supplements are associated with lower breast cancer survival and may hasten the spread of lung cancer.

The simple answer is: "The only one who can answer that question is your oncologist."

A better answer is: "Ask your oncologist before you take any type of supplements, but check out some of the considerations below—the reasons for and against—so you can better understand your healthcare provider’s answer and decide together.”

Never take a vitamin, mineral, or antioxidant supplement without first consulting your healthcare provider, as this can be dangerous for some people with cancer.

This article covers some of the possible risks and benefits of supplements, but it's important to note a few things. There are many types of cancer, and even with a single type of cancer there are tremendous differences. Add that to each unique person, with their own body characteristics and other medical conditions, and it's easy to see why even something that may seem like a simple question is very complex.

A list of vitamins and minerals needed for health, as well as a list of common dietary antioxidants, can be found at the end of this article.

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Reasons Your Oncologist May Recommend Not Taking Vitamins

There are a number of reasons why your oncologist may recommend that you avoid vitamin or mineral supplements. Sometimes the particular reason may not be obvious to everyone (such as knowing the vitamins that may affect blood test results with cancer), and it's important to not only ask your healthcare provider what you should avoid, but also why. Some reasons to avoid vitamin supplements may include:

Possible Interference With the Benefits of Treatment

One of the main reasons oncologists often don't recommend vitamin and mineral supplements or antioxidant formulas is because they could counteract the effects of chemotherapy or radiation therapy. Free radicals in our bodies (produced by substances such as tobacco smoke, radiation, and normal metabolic processes) can damage the DNA in our cells (mutational damage which can lead to cancer.) This damage is called "oxidative damage" since the reaction involves oxygen. Antioxidants produced by our bodies and ingested in our diets work by essentially neutralizing these free radicals and preventing oxidative damage; thus protecting cells.

Antioxidants may protect cancer cells from being damaged by chemotherapy and radiation therapy. We don't want to "protect" cancer cells.

This theory is supported by a 2019 study published in The American Journal of Clinical Nutrition. In this study, postmenopausal women who too antioxidant supplements during chemotherapy and radiation therapy for breast cancer had poorer recurrence free survival and a higher mortality risk (were 64% more likely to die).

A 2019 study also found antioxidant supplements may promote the spread (metastasis) of lung cancer.

Interaction With Chemotherapy

There are some studies, especially in people who smoke, in which people using supplements had worse outcomes. A 2008 study showed that vitamin C supplements decreased the effectiveness of chemotherapy by 30 to 70 percent among human leukemia and lymphoma cells in a laboratory setting. Other studies suggest that high-dose vitamin C may have benefits for people treated for cancer—at least in the lab.

Vitamin C and Hormonal Therapy

Studies looking at human breast cancer cells in a lab have found that vitamin C decreased the effectiveness of tamoxifen. In these studies it was thought that vitamin C interfered with apoptosis, that is, cell death, in cancer cells.

Vitamin C and Targeted Therapy

Targeted therapies are a newer treatment for cancer that target specific pathways in the growth of cancer cells. In animal studies, vitamin C appeared to reduce the anti-cancer activity of the targeted therapy Velade (bortezomib). Velcade is used for people with multiple myeloma and some types of lymphoma.

Some of the risk may be more theoretical. A 2007 review of studies from 1966 to 2007 found no evidence that antioxidant supplements interfered with chemotherapy, and some researchers believe that antioxidants may help to protect normal cells without interfering with the effectiveness of cancer therapy. This review included studies using glutathione, vitamin A, vitamin C, vitamin E, ellagic acid, selenium, and beta-carotene and concluded that antioxidants may improve tumor response to treatment and survival rates, in addition to aiding patients' tolerance to treatment.

Another systematic review of 33 studies found evidence that using antioxidants with chemotherapy resulted in less toxicity, which in turn allowed people to complete full doses of therapy. The exception was one study which showed an increase in toxicity in those using a vitamin A supplement. This review evaluated studies using N-acetylcysteine, vitamin E, selenium, L-carnitine, Coenzyme Q10, and ellagic acid.

Interaction With Other Medications

There are many examples of possible interactions, but a simple example is that of vitamin E potentially increasing the risk of bleeding in people who are taking the blood thinner Coumadin.

Interactions That Affect Blood Tests

Some vitamins such as biotin (vitamin B7), may interfere with the assays done to provide some laboratory results. Of note is that biotin may be present in many combination vitamin supplements.

Dietary Sources vs Supplements

We don't have as many studies looking at the use of antioxidants during cancer treatment, but the use of these supplements with the goal of cancer prevention has revealed some interesting findings. For example, seeing how a high dietary intake of beta-carotene was associated with a lower risk of developing lung cancer, a large study looking at the use of a beta-carotene supplement found that the risk of lung cancer was actually increased. Similar were findings with prostate cancer, in which dietary vitamin E is associated with a lower risk, but a study evaluating vitamin E supplements found an increased risk.

In general, the rule for getting vitamins and minerals during cancer treatment is "food first."

Theories have been proposed to explain this. Perhaps there are phytochemicals (plant-based chemicals) in food aside from beta-carotene that are responsible for the cancer prevention properties. Another theory that's been proposed is that receiving one antioxidant as a supplement may result in the body absorbing less or using less, of another important antioxidant.

Sometimes taking a supplement for one concern may raise another concern. An example is a study in which people with melanoma were treated with selenium. Researchers found that the supplement was associated with a lower risk of developing a second cancer in the lungs, colon, or prostate, but was also associated with an elevated risk of diabetes.

Most oncologists recommend eating a healthy diet and do not believe the antioxidants derived from foods pose a threat to the effectiveness of cancer treatments.

Method of Study

Interpreting some of the information on antioxidants during cancer treatment is difficult for many reasons, one of which is the different methods used. Some studies are done on rodents, and effects in rodents may or may not be the same as those in humans. Many of these studies have been done on human cancer cells grown in a dish in a lab. While this does give us some good information, it doesn't take into account the myriad of other processes going on in the human body that could alter the response seen in the lab. Vitamins and antioxidants may have different effects on normal cells and cancer cells. For example, in lab studies cancer cells appear to take up vitamin C better than normal cells.

There are also special cases that can't be addressed when studies look at the general population. For example, vitamin C in people with glucose-6-phosphatase deficiency can lead to hemolysis. People with hemochromatosis are at risk of iron overload with iron supplements, and more. The role of vitamins and antioxidants can also vary depending on particular treatments. For example, vitamin C may reduce the effectiveness of radiation but may also reduce the toxicity.

Reasons Your Oncologist May Recommend Vitamins  

Just as there are several reasons why people with cancer may wish to avoid vitamins, there are a number of reasons why they may be recommended instead. Some of these include:

Nutritional Deficiencies

With side effects of loss of appetite and nausea common with cancer, nutritional deficiencies are not uncommon. Talk to your healthcare provider about this possibility. Some researchers have theorized that supplementation could help reduce cancer cachexia. Cachexia is a syndrome of unintentional weight loss, muscle wasting, and decreased appetite that affects up to 50 percent of people with advanced cancer. It's felt that cachexia contributes directly to 20 percent of cancer deaths. Sadly, with the exception of fish oil which might help, nutritional supplements have not been found to aid in this syndrome.

To Prevent a Second Cancer

Since cancer treatments such as chemotherapy and radiation therapy predispose survivors to other cancers, it has been hoped that the risk of a second cancer could be reduced with the use of antioxidant supplements. As noted above, in one study people with melanoma treated with selenium had a lower later risk of developing lung, colon, or prostate cancer (but with an increased risk of diabetes.) Just as supplements (as opposed to dietary antioxidants) have not shown consistent results in preventing cancer, there is not much evidence that these supplements would be helpful in preventing a second cancer in survivors.

To Decrease Toxicity of Treatments

Studies have been mixed with regard to antioxidants increasing or reducing the toxicity of chemotherapy, but some research suggests they may improve quality of life for some people during cancer treatment. In one study, an antioxidant mixture containing vitamin C, vitamin E, melatonin and green tea extract was found to decrease fatigue in people with pancreatic cancer.

People With Advanced Cancer and/or Cachexia

 An oft-quoted study supporting the use of vitamin supplements during cancer treatment showed an increase in the length of survival. This 2009 study revealed a longer than expected median survival time, with 76% of the patients longer living longer than was predicted (median increase in survival 5 months.) It’s important to note that this was a very small study (41 patients) conducted with people considered to have end-stage cancer who had a predicted life expectancy of only 12 months. These patients were treated with a supplement of coenzyme Q10, vitamins A, C, and E, selenium, folic acid, and for those without lung cancer, beta-carotene.

Cancer cachexia is extremely challenging to treat, but there is some evidence that omega-3-fatty acids can be helpful.

The Special Case of Vitamin D and Cancer

For several reasons, vitamin D deserves special consideration with regard to its role in cancer treatment.

The first reason is that it may be difficult to get an adequate amount of vitamin D by dietary measures. While the recommended daily allowance is 400 to 800 IU's per day depending upon age, studies looking at cancer prevention have looked at higher numbers-- up to 1000 to 2000 IU's per day. We think of fortified milk as being a source of vitamin D, but at 100 IU's per glass it would require drinking 8 glasses per day just to reach the 800 IU's recommended for a 70-year-old man or woman (much less the dose studied in cancer prevention studies.) The sun is a good source of vitamin D, requiring only short amounts of time with arms and face exposed to absorb upwards of 5000 IU's. That is, if you live in a region where you can be outside with your arms and face exposed and if the angle of the sun at your latitude allows the absorption of vitamin D-producing rays.

This can be a problem in northern climates.

For this reason, many healthcare providers recommend a supplement of vitamin D3. Who should be taking a supplement? Thankfully your healthcare provider has a simple way of determining this. A simple and inexpensive blood test can provide you and your healthcare provider with a measure of your blood level of vitamin D (rather, the body's breakdown products). While this won't tell you what your total body "store" of vitamin D is, it can be used to determine if a supplement is needed and guide treatment. Keep in mind that the majority of people in the United States are vitamin D deficient.

Why is this important?

There have been many studies that have evaluated the role of vitamin D both in cancer prevention and in cancer treatment. Low blood levels of vitamin D are associated with an increased risk of several cancers, and a high vitamin D level is associated with a lower risk of developing breast and colorectal cancer. People who have higher vitamin D levels at the time of diagnosis have longer survival rates from lung cancer than those with lower levels. And, regarding our question about using vitamins during cancer treatment, low vitamin D levels appear to increase the risk of breast cancer spreading (metastasizing). Perhaps the most dramatic effect has been seen with colon cancer. A large National Cancer Institute study found that people with high vitamin D levels were 76 percent less likely to die from their disease than people with low levels of the vitamin.

Since some cancer treatments predispose to osteoporosis, and vitamin D aids calcium absorption, an adequate vitamin D level may also affect quality of life for some cancer patients.

Vitamin D is not an antioxidant. It actually functions more like a hormone than a vitamin in the body.

Even though much of the research shows a positive role of vitamin D for at least some people with cancer, it's very important to talk to your healthcare provider before using a supplement. In fact, your healthcare provider will want to monitor your levels to see if they change if you begin a supplement. The normal range of values may not be the ideal range for someone with cancer. For example, at the Mayo Clinic in Minnesota, the normal range for vitamin D levels is 30-80. Yet some studies suggest that a level of 50 is better than a level of 31.

Vitamin D supplementation is not for everyone. There are potential side effects including a very painful one—kidney stones—if levels are too high.

Vitamin B12

Like vitamin D, vitamin B12 is not an antioxidant, and it's thought that many people over the age of 50 are deficient. But as with other vitamins, it's important to talk to your healthcare provider.

Taking a Vitamin or Supplement Recommended by Your Healthcare Provider

If your oncologist does recommend a supplement, there are a few things to keep in mind.

  • Again, only use vitamins or minerals (or any other nutritional or herbal supplements) if your oncologist does not think these could be harmful in your particular situation.
  • The potential risks and benefits noted above are just a glimpse to demonstrate why vitamin and mineral supplements need to be considered individually for each person. Your healthcare provider may recommend using or avoiding vitamin or mineral supplements for additional reasons.
  • Always bring all over-the-counter medications and supplements with you to each oncology appointment.
  • Don't be fooled by advertising that says "natural" or "plant-based." An example is hemlock. It is natural, it is plant-based, and it can even be grown organically. Yet it's thought by many that it was the poison referred to in Romeo and Juliet. In fact, many strong chemotherapy drugs are plant-based.
  • Use only the recommended dose. More isn't necessarily better and could be harmful. Keep in mind that supplements often contain levels of vitamins and minerals which far exceed what you would get eating a normal diet. In addition, there is little regulation of these supplements in the United States, and some products could contain traces of pesticides and heavy metals.
  • Use only good quality vitamins and minerals. Check to see if they have been evaluated by Also check to see if there is a USP or NF seal on the label, indicating that quality control testing has been done on the product.
  • Continue to discuss your intake of vitamins and minerals at each follow-up visit. Many clinical trials are in progress, and information could change—in either direction—as we learn more.

Review of Vitamins and Minerals

Vitamins Our Bodies Need:

  • Vitamin A
  • Vitamin B6 (pantothenic acid)
  • Vitamin B12 (biotin)
  • Vitamin D
  • Vitamin E
  • Vitamin K
  • Folic acid
  • Niacin
  • Riboflavin
  • Thiamine

Minerals Our Bodies Need:


Antioxidants may be vitamins, minerals, or other nutrients. Examples of these include:

  • Vitamin A
  • Vitamin C
  • Vitamin E
  • Selenium
  • Carotenoids such as beta-carotene and lycopene
2 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Jung, A., Cai, X., Thoene, K. et al. Antioxidant Supplementation and Breast Cancer Prognosis in Postmenopausal Women Undergoing Chemotherapy and Radiation Therapy. The American Journal of Clinical Nutrition. 2019. 109(1):69-78. doi:10.1093/ajcn/nqy223

  2. Lignitto L, LeBoeuf SE, Hamer H, et al. Nrf2 Activation Promotes Lung Cancer Metastasis by Inhibiting the Degradation of Bach1. Cell. 2019. doi: 10.1016/j.cell.2019.06.003

Additional Reading

By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."