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Are Dietary Supplements Safe?

Used properly, certain dietary supplements may help reduce the risk of some diseases. Some might also reduce discomfort caused by certain conditions or medicines, or they might help you feel better, which can improve your quality of life. Most people can use dietary supplements safely, as long as they don’t take too much. But dietary supplements are not totally safe, and taking them can have risks, especially for people who are getting cancer treatment.

Risks and side effects of dietary supplements

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. However, most people who have side effects, illnesses, or drug interactions from dietary supplements don’t call a poison control center or the supplement manufacturer. This means that the numbers we have are likely lower than what actually happens.

Side effects from dietary supplements happen most often if people take high doses or use them instead of medicines prescribed by their health care provider.

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.

Taking many different supplements can increase the risk of side effects and drug interactions.

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. If you are getting radiation treatments, talk to your doctors before taking any supplement.
  • For people who need surgery, some supplements may react with medicines used during and after surgery or might increase the risk of certain side effects such as bleeding and infection. Others might affect the way the heart and brain work, which might lead to complications during and after surgery. Let your doctor know what supplements you are taking before you have surgery.
  • 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, which 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 doctor or cancer care team. They can help you understand whether or not a particular product might be safe for you.

Why it’s hard to tell if dietary supplements are safe and effective

Like medicines, dietary supplements have risks and side effects. But unlike medicines, most people choose which dietary supplements to take without discussing them with their doctor, nurse, or pharmacist.

Much of what you hear or read about dietary supplements is based on anecdotal evidence. Anecdotal evidence is based on a person’s (even a doctor’s) personal experience or opinion, rather than research. Be skeptical of sources that make big claims based on a few people’s testimonials or vague references to “scientific proof.” The rule “if it sounds too good to be true, it probably is” usually applies to such claims. And keep in mind that the makers and sellers of supplements have a financial interest in promoting their products.

Dietary supplement advertising and promotion

Supplement makers do not have to get approval from the FDA to sell their products. The FDA does watch 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. As its resources permit, the FDA looks at supplement labels and other information, such as package inserts, claims, and online ads. But it cannot review all of the many products on the market today.

And remember - no matter what a supplement maker claims, dietary supplements are not intended to treat, diagnose, cure, or relieve the effects of diseases.

To avoid unsafe supplements, don’t buy:

  • Products that claim to work like prescription medicines – anything that claims to treat an illness or cure a medical condition
  • Products that are advertised through mass e-mails
  • Products that promise weight loss, body-building, or enhanced sexual performance
  • Products that say they are a legal alternative to anabolic steroids

Possible quality issues with some dietary supplements

Many dietary supplements are made under careful conditions and labeled correctly. But others are not made as carefully. 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 ingredient. Some even contain chemicals that could be harmful to certain people. And some supplements contain a larger dose than the label lists. Serious illnesses and even deaths have resulted from these kinds of problems.

Common misconceptions about dietary supplements

"Natural is safer and better"

It is a common belief that natural is safer or better than man-made or refined substances. Not only is this not always true, but some natural products can be harmful even if used as directed. For instance, comfrey and kava can harm the liver.

Botanical supplements (such as garlic, ginger, ginkgo biloba, echinacea, and others) are made of plant material, so many of them are sold as “natural” products. But plants themselves are made up of many chemicals. Even different parts of the same plant can contain different chemicals. Some of these might be helpful, while others might be poisonous or cause allergies in humans. Botanicals that are marketed as “all natural” are not always the most helpful ones since they may not be refined to remove potentially harmful chemicals. Natural products can also be grown under different conditions (such as in different soils), which might also affect the levels of some chemicals. This can make it harder to control exactly what's in the final product.

"More is better"

Some people believe that mega-doses of certain vitamins can prevent or cure diseases. However, no scientific studies have proven this to be true. In fact, large doses of some vitamins or minerals can be dangerous and even harmful. 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 health care team before taking large doses of any vitamin, mineral, or other supplement. Your doctor, nurse or pharmacist should be able to help you find information on safe dosages.

"It’s been used for hundreds/thousands of years, so it must work"

Knowing that a botanical has been used in folk or traditional medicine for many years can be helpful, but it is not proof that it works or that it’s safe. If small amounts of a plant caused painful or life-threatening side effects right away, it probably wouldn’t have been used for very long. But in the distant past, scientific methods weren't used to look for possible long-term side effects. So, if a plant seemed useful over the short term but actually increased the risk of a chronic disease (like cancer, heart failure, or kidney failure), that side effect likely would not have been noticed.

Also remember that most herbs, plants, and other methods were used in traditional medicine systems to reduce symptoms or make the person feel better. This was helpful to people who were likely to recover anyway. But if a person died, it was accepted that death was 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 can’t hurt to take supplements along with my regular medicines"

Many people assume that it is safe to take dietary supplements along with prescription medicines. This is not always true. 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. Most medicine companies and producers of herbal supplements do not research possible medicine interactions, so the risks of taking supplements with other medicines are largely unknown.

Talk with your health care team about any supplements you are taking or are thinking about taking. Your doctor or pharmacist can tell you about any known interactions with medicines you may be taking. Keep in mind that with new medicines and supplements, interactions may not yet be known.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as editors and translators with extensive experience in medical writing.

Supplement Your Knowledge: Dietary Supplement Education Initiative 

This site includes fact sheets and videos about dietary supplements for the public, educators and healthcare professionals. Materials for the public are available in English and Spanish.

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National Cancer Institute. Cancer Therapy Interactions with Foods and Dietary Supplements. Accessed at on August 18, 2021.

National Center for Complementary and Integrative Health. Dietary and Herbal Supplements. Last updated February 2020. Accessed at on May 27, 2021.

Office of Dietary Supplements, National Institutes of Health. What You Need to Know: Dietary Supplements. Accessed at on May 11, 2021.

Last Revised: September 6, 2023

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