Nutritional supplements and cancer

Why cancer patients take supplements?

In general, cancer patients take supplements for some or all of the following reasons:


  • Herbal medicines sold as supplements claimed to have the ability to fight cancer
  • Antioxidants that protect cells from oxidative damage
  • Supplements said to enhance the immune system and/or to reduce inflammation
  • Supplements claimed to improve the effects of conventional medicine.

It is very important to understand the differences between a supplement and drug to avoid misinformation when buying supplements.

Differences between medication and supplements

Drugs need to be manufactured under Good Manufacturing Practices (GMP). This means that they are manufactured under carefully monitored conditions and packaged with complete information on the best dosage routine and schedule. The package information must include a list of all known side effectscontraindications (special conditions under which using the drug could be dangerous), or possible unsafe interactions with other drugs. There is strict control of the raw initial ingredientsintermediates and of the end product.

A system that addresses any chemical, biological and physicals hazards in the food industry is Hazard Analysis and Critical Control Points (HACCP). Supplements need to be manufactured according to the HACCP standards. This implies that possible contamination needs to be checked and it must be ensured that there are strict hygiene procedures during the production process. The main differences between GMP and HACCP is no monitoring regarding the end product of HACCP. This means that the minimum and maximum tolerable content of the active ingredient(s) in the end product is not monitored and, consequently, not guaranteed. In contrast, this control is required for registered drugs. Moreover, there is also no stability test of HACCP verifying whether or not the supplement remains stable until the expiry date and/or at higher temperature/humidity, which is obligatory for medicines.

Supplements bought from an unknown source through certain manufacturers and some websites could be contaminated with germs, pesticides, toxins, toxic heavy metals, or residual solvents. They could also contain more, less, or even none of the purported substance/s mentioned in the product information. Some herbal supplements were even found to contain prescription drugs. These product quality issues could have serious consequences for the health of the patient or other consumers. A pharmacist is held responsible for the supplements he/she sells, while the purchaser is solely responsible for the supplement purchased online and from other sources.

There are some clinical trials with food supplements in cancer, but these are sporadic and not of the same extent as the clinical trials performed with registered drugs. As a consequence, without extensive clinical trials, efficacy claims made for supplements are often false or exaggerated.

Websites often report on promising cancer research but do not mention clearly that the research has been performed on cancer cells only, not in people. Besides, at high doses almost everything kills cancer cells. A treatment should only be postulated to show promise once it has been tested in people. If you read news reports about spectacular cancer treatments, fully evaluate any published research findings to learn if and how accurate, these promising study results were achieved.

Malnutrition in cancer patients

In the course of their disease, many cancer patients develop tumor-associated malnutrition which, among other things, is characterized by a micronutrient deficit. In practice, it is not always possible in such special circumstances to counteract this condition in an optimum way by the diet, i.e. the conventional food selection. Often the situation is made more difficult by the limited dietary pattern of tumor patients which may lead to a very one-sided nutrition. Therefore, it is not surprising that the use of nutrient supplements, among them especially vitamins and mineral substances, is widespread among cancer patients.

Malnutrition is very prevalent in cancer patients, ranging from 40-80% of cases. Cancer can alter metabolism, which increases dietary needs for more proteins, carbohydrates, fat, vitamins, and minerals. Malnutrition can also be caused by the side effects of cancer treatment. In addition, malnutrition leads to increased toxicity to cancer treatment that lowers quality of life and worsens prognosis.

Vitamin and mineral deficiencies can be mostly traced through a blood analysis. Your GP (General Practitioner) can advise you on the vitamins and minerals that need to be supplemented and at what dosage. Discuss the use of supplements during chemo- or radiotherapy with your oncologist as some supplements may interact with conventional treatment making it less efficacious (1).

Interaction between treatment and supplements

Adding large amounts of antioxidants to our diet seems to prevent tissue and organ damage as it protects our cells. But the presence of antioxidants in the diet during cancer therapy is very controversial since radiation therapy, and to a lesser extent chemotherapy, acts by means of creating ROS to kill cancer cells. Therefore, in theory, antioxidants during radiotherapy or chemotherapy might inhibit the efficacy of the treatment through neutralization of the ROS. There are several clinical trials ongoing in an attempt to clarify this controversy, but no firm conclusions have been drawn yet. Some clinical trials suggest that antioxidant supplementation decreases the side effects and enhances the efficacy of conventional treatment, yet other clinical trials indicate a shorter survival time in patients taking antioxidants during radiation therapy.

A non-exhaustive list of supplements and foods containing large amounts of antioxidants that are often used by cancer patients is provided below.

  • Vitamin A, C, E, beta-carotene, lycopene, lutein
  • Selenium, zinc
  • Coenzyme Q10, glutathione, melatonin, resveratrol
  • Berries (blueberries, strawberries, raspberries, etc.), curcumin, green tea

Cancer patients often seek treatments to improve their immune system defense mechanisms in order to tolerate chemotherapy in a better way and to avoid infections on top of their illness. Some cancer patients also try to improve their immune system with the goal of provoking anticancer activity. Although many plants could possibly act as immunostimulants, for many of those plants proof is lacking. Caution should be used when considering taking supplementation with immunostimulating herbs. These herbs could also be toxic or interfere with standard treatment. Check the quality assurance activities of the supplier.

Some clinical trials with probiotics in cancer patients have been performed in recent years. The effect of probiotics on diarrhea during radio- or chemotherapy has mainly been investigated. All trials reported positive results with a decrease in diarrhea and an improvement in stool consistency. Since these were all small-scale trials, results need to be confirmed in large-scale controlled trials (2).

No matter what they claim, dietary supplements are not intended to treat, diagnose, cure, or relieve the effects of diseases. They cannot completely prevent diseases, as some vaccines can. But some supplements are useful in reducing the risk of certain diseases. They are allowed to make label claims about these uses (3).

The risk of side effects of supplements

Like drugs, dietary supplements have risks and side effects. But sellers aren’t required to do research studies in people to prove that a dietary supplement is safe. And unlike drugs, dietary supplements are mostly self-prescribed with no input from informed medical sources like doctors, nurses, or pharmacists.

Used properly, certain dietary supplements may help reduce the risk of some diseases, reduce discomfort caused by certain drugs or conditions, or simply make you feel better (improve your quality of life). And most people can use dietary supplements safely within certain dosage guidelines. But taking dietary supplements can be risky, especially for people who are getting cancer treatment (4).

“We tell patients that if you are on any chemotherapy or undergoing radiation, or planning for it in the future: no herbs, no antioxidants, no dietary supplements, across the board. Particularly herbal agents, because they can interact with and decrease the level of chemotherapy or any medication that enters your body,” explained Dr. Cassileth.

These effects are due to pharmacokinetic interactions—what happens when biologically active compounds in an herb alter the way a chemotherapy drug is absorbed, distributed in the body, metabolized, or eliminated. These interactions can happen for many reasons, including interference with the enzymes in the liver that normally break down the drugs, or interactions with the transporters that carry drugs across cell membranes. St. John’s Wort, garlic extract, and Echinacea are examples of commonly used herbal products thought to pharmacokinetically interact with chemotherapy drugs.

Pharmacokinetic interactions can have two potentially disastrous consequences. One is that less chemotherapy drug circulates in the bloodstream than is needed, leading to treatment failure. The other is the opposite effect: if the chemotherapy drugs are not broken down and removed from the body as expected, severe side effects can occur as a result of an overdose (5).

Tell your physician you take supplements

Patients often do not report their use of supplements to their provider. As a result, there is a gap in communication between the providers and their patients. This gap in communication may result from (a) patients’ perception that their physicians are indifferent or negative toward complementary therapies or (b) physicians’ emphasis on scientific studies and evidence-based medicine, rather than patient references, in the selection of such therapies.

It is interesting to note that research showed that a high desire for control of ones disease was only associated with increased use of dietary supplements and did not predict making changes in diet or physical activity, which may be motivated more by other psychosocial factors such as belief in a diet-disease connection or concerns about weight changes. It is possible that taking supplements may be a coping behavior for dealing with the emotional aftermath of cancer rather than a health improvement strategy (6).


Patients frequently see natural product consumption as an avenue that they can use to empower themselves, attempt to take control of their health, and increase their quality of life (7).

This article has been written in collaboration between Anticancer Fund and Esperity



(1) Ströhle A, Zänker K, Hahn A, Nutrition in oncology: The case of micronutrients (Review). Oncol Rep. 2010 Oct;24(4):815-28.


(3). American Cancer Society – Dietary supplement advertising and promotion

(4) American Cancer Society – Risks and side effects of dietary supplements

(5). National Cancer Institute – Dietary Supplements and Cancer Treatment: A Risky Mixture

(6) Patterson RE, Neuhouser ML, Hedderson MM, et al. Changes in diet, physical activity, and supplement use among adults diagnosed with cancer. J Am Diet Assoc. 2003 Mar;103(3):323-8.

(7) Frenkel M, Abrams DI, Ladas EJ, et al. Integrating dietary supplements into cancer care. Integr Cancer There. 2013 Sep;12(5):369-84. doi: 10.1177/1534735412473642.