Are Dietary Supplements Safe?
When used properly, certain dietary supplements may help reduce disease risk and improve wellbeing. But they can have risks for people who are getting cancer treatment. Before using dietary supplements during cancer, learn the facts to make an informed and safe decision.
It's very important to talk to your cancer care team about any supplements you take or plan to take.
What are dietary supplements?
The term "dietary supplement" refers to a wide range of products including vitamins, minerals, herbs and other botanicals, amino acids, enzymes, and more. You can find dietary supplements in many forms, such as pills, gummies, powders, liquids, teas, and bars.
Dietary supplements do not need to be tested and approved by the United States Food and Drug Administration (FDA) since they are not intended to treat, diagnose, prevent, or cure diseases.
Why it’s hard to tell if dietary supplements are safe and effective
Dietary supplements don't have the same strict safety requirements that medicines do. In the US, medicines are regulated by the FDA. They usually must go through many tests (clinical trials) to make sure they’re safe and work well. But dietary supplements do not have to be tested. Dietary supplements can be sold if the maker doesn't claim they can treat, diagnose, prevent, or cure a disease.
Also, much of what people hear or read about dietary supplements is based on personal experience or opinion, rather than research. While personal stories can be compelling, it’s important to pair them with scientific evidence when making health decisions. And keep in mind that the makers and sellers of supplements have a financial interest in promoting their products.
Regulations for dietary supplements
Be aware that some companies don’t follow the FDA’s rules about making claims and labeling supplements correctly. In some cases, when herbal supplements have been tested, they have been found to contain very little or none of the listed ingredients. Some even contain chemicals that could be harmful to certain people. And some supplements contain a larger dose than the label lists. This is why it’s important to choose high-quality supplements.
Although supplements are not required to go through clinical trials, companies that make them are supposed to:
- Have data showing the products are safe
- Make sure that the details on product labels are truthful
- Avoid making false claims
- Correctly show what is in the supplement
The FDA watches for reports of illness, injury, or reactions to supplements. All supplement makers are supposed to report harmful effects to the FDA. The FDA also wants consumers to report reactions or other problems with supplements.
Finding high-quality dietary supplements
Companies that make dietary supplements must follow Good Manufacturing Practices (GMPs). The GMPs state that dietary supplements must:
- Be produced in a quality manner
- Not contain any contaminants or impurities
- Be labeled with the ingredients that are actually in the product
There are several organizations that provide quality testing. They ensure the ingredients listed on the label are actually in the product and that it doesn’t contain any contaminants or harmful chemicals. When choosing dietary supplements, look for seals of quality assurance from organizations that offer quality testing like*:
- US Pharmacopeia (USP)
- NSF International
- ConsumerLab.com
*Inclusion on this list does not imply endorsement by the American Cancer Society
For more tips on selecting safe dietary supplements see How to Use Integrative (Holistic) Therapies Safely.
Understanding the claims on dietary supplement labels
Before you buy a dietary supplement, it’s important to read the label carefully. Look at the claims, packaging, ingredients, and directions for use. It can be easy to misread the claims that are being made about products.
The FDA watches for products that may be unsafe or make false or misleading claims. But they can only do this after the product is already on the market. The FDA does not have the resources to review every product on the market.
Watch out for structure or function claims
Structure or function claims are statements about the effect of the dietary supplement on the structure or function of the body. These claims are not reviewed by the FDA and are easy to misinterpret.
Some examples are:
- “Works as an antioxidant”
- “Promotes normal urinary flow”
- “Supports normal blood glucose”
Statements like these may lead people to believe that the product has been proven to prevent or treat diseases. This is not the case. Don’t assume that because a product claims to support or promote healthy body function that it prevents or reduces the risk of any disease, including cancer.
Common misconceptions about dietary supplements
It is not always true that natural products are safer or better than synthetic substances. Some natural products can be harmful even if used as directed.
Many botanical supplements (such as garlic, ginger, ginkgo biloba, echinacea, and others) are sold as “natural” products. But plants themselves are made up of many chemicals, some helpful and some harmful. For instance, comfrey, green tea, and kava can harm the liver. Even different growing conditions (such as in different soils) might affect the levels of some chemicals. This can make it harder to control exactly what's in the final product.
Dietary supplements, herbal preparations, and homeopathic treatments caused almost 70,000 calls to US poison control centers in 2019. Of these calls, more than 7,000 people needed treatment at health care facilities. More than 800 of these cases led to moderate to severe outcomes. And these numbers are likely lower than what actually happens.
Some people believe that mega-doses of certain vitamins can prevent or cure diseases. However, no scientific studies have proven this to be true. Side effects from dietary supplements happen most often if people take high doses. For example, the body cannot get rid of large doses of vitamin A. It can reach toxic levels when too much is taken, which can damage organs and interfere with certain medicines.
Talk with your cancer care team before taking large doses of any vitamin, mineral, or other supplement. They can help you find information on safe dosages.
While traditional medicinal use can help offer helpful insights, it does not guarantee that these methods will work or are safe. Most traditional remedies were used to reduce symptoms, not cure illness. This was helpful to people who were likely to recover anyway. But death was accepted as a possible outcome of most serious illnesses.
Finally, keep in mind that many of these methods were used in the distant past because they were the best option available at the time, as more effective treatments had not yet been developed.
It is not always true that it is safe to take dietary supplements along with prescription medicines. For example, certain dietary supplements can block or speed up the body’s ability to break down some medicines. This can cause a person to have too much or too little of the medicine in their bloodstream.
Some supplements can also cause problems if taken along with certain medicines. For example:
- Antioxidants, such as vitamins C and E, might make some chemotherapy medicines less effective.
- Vitamin K can make the blood thinner, warfarin, less effective and increase the risk of blood clotting.
- St. John’s wort can make certain medicines less effective. Some examples are birth control pills and medicines for depression, heart problems, HIV, and cancer.
Talk with your cancer care team about any supplements you are taking or are thinking about taking. Your cancer care team can tell you about any known interactions with medicines you may be taking. Keep in mind that interactions between new medicines and supplements may not yet be known.
Special concerns for people being treated for cancer
There are several ways that supplements can cause problems for people during cancer treatment. For example:
- Some dietary supplements can cause skin sensitivity and severe reactions when taken during radiation treatment.
- Some supplements may react with medicines used during and after surgery. Or they might increase the risk of certain side effects or complications.
- People getting chemotherapy (or other cancer medicines) may be at higher risk for side effects if they take some dietary supplements. Some supplements can interfere with how the body breaks down these drugs. This can make them less effective or increase side effects.
Because of these concerns, many cancer experts advise people to avoid dietary supplements while getting cancer treatment. But if you decide to take supplements, be sure to discuss this with your cancer care team before any treatment. They can help you understand whether a product might be safe for you.
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- References
Developed by the American Cancer Society medical and editorial content team with medical review and contribution by the American Society of Clinical Oncology (ASCO).
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Last Revised: July 11, 2025
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