Dietary Supplements: The Basics

Author: Marisa Healy, BSN, RN
Content Contributor: Carolyn Vachani, RN, MSN
Last Reviewed: August 28, 2023

Many people take one or more dietary supplements without thinking about the safety of using these products. They are often promoted as “natural" but this can be misleading. It is important to remember that “natural” does not mean safe. There is also quite a bit of confusion as to how dietary supplements are "regulated" by the U.S. Food & Drug Administration (FDA).

What is a dietary supplement?

The U.S. FDA categorizes vitamins, minerals, amino acids, herbals, and other botanicals as dietary supplements (DS). The DS category also includes products like hormones (such as melatonin), glucosamine, chondroitin, probiotics, fish oil, and SAMe. You can tell that an item falls under the DS category because the ingredient label will say "Supplement Facts."

Dietary supplements can come in many forms, including tablets/capsules, powders, oils, drinks, or edible items like chews, gummies, or bars.

Are dietary supplements helpful?

It is best to get nutrients from their natural source (your food!). Your body can better absorb nutrients from food sources. In addition, there are other beneficial nutrients in many foods. For example, fruits and vegetables contain vitamins, minerals, fiber, and water. Some patients may benefit from the use of a supplement depending on their cancer treatment or if they have a known deficiency (low level). Ask your provider if you would benefit from the use of a supplement.

In general, a DS is intended to supplement your diet. They are not intended to treat, prevent or cure diseases. The manufacturer should not make any claims on their labels or marketing to suggest that they do.

What does the FDA regulate?

Consumers are often surprised to learn that dietary supplements are less tightly regulated than prescription or over-the-counter medications, food additives, and even the food we buy. The FDA cannot review DS products for safety or effectiveness before they are sold.

It is the responsibility of the company making the DS to assure the safety and quality of their products. They must be sure that any claims made about the supplement are not false or misleading. The manufacturer is responsible for establishing its own manufacturing guidelines using clean and consistent practices. They must ensure that what is on the label is what is actually in the product. This means they do not have any contaminants or impurities and it has the amount the label states.

If a user has a serious problem, the manufacturer must report it to the FDA. The FDA can take a product off the market if it is found to be unsafe or if the product marketing is false or misleading.

What are the risks of taking dietary supplements?

While many DS compounds are “natural,” this does not mean they are safe. These compounds can have strong effects on the body and can interfere or interact with the medications you are taking. For example:

  • Some supplements can decrease how well the commonly used blood thinner Coumadin works.
  • St. John’s wort can reduce how well many medications work because it causes them to be broken down faster in the body – this includes many oral chemotherapy medicines.

Special concerns for people getting cancer treatment

Dietary supplements can cause problems while receiving cancer treatments. Antioxidants (Vitamins A, C, E, beta-carotene, selenium and others) may interfere with how chemotherapy and radiation kill cancer cells. Supplements can interact with cancer treatments and other medications, causing them to be absorbed differently. This can either make blood levels higher, leading to more side effects, or lower, causing the medication or treatment to be less effective. Some can increase the risk for bleeding, which is a concern for many people with cancer who are already at higher risk of bleeding.

Most cancer care providers recommend not taking dietary supplements during treatments. If you choose to take them, make sure your care team is aware of what you are taking to screen for possible interactions and side effects. They will include them on your medication list.

How can I choose a quality DS product?

You should start by buying products from reputable retailers and manufacturers.

There are a few independent organizations that test products to check for proper manufacturing techniques and to ensure that what is on the label is what is actually in the product. If a company’s product passes their testing, they are given a seal of approval from the organization. These organizations include:

If you notice a side effect or reaction to a DS, you should report it to the manufacturer and to the U.S. FDA.

Should I tell my healthcare team about any dietary supplements I use?

Absolutely! Surveys have shown that many patients don’t tell their healthcare teams about supplement use. They may not think it is important or may think that their providers will not understand. Look to reliable resources for information about possible interactions and side effects (see resources below). Talk with your pharmacist about possible interactions with medications or foods.

If you have any literature on what you are taking, bring it to your appointment for your provider to see. Tell them why you are taking it and talk about possible effects on your other medications and treatments. Some supplements may interfere with cancer therapies, affecting how well they work or actually making certain side effects worse. Some can increase the risk of bleeding, which is a concern if your platelet count is low. And unfortunately, some are a sham, promoted by retailers preying on cancer patients.

Your healthcare team can help you make an educated decision about which supplements you may need or want to use.

Resources for more information

National Center for Complementary and Alternative Medicine

The U.S. Federal Government's lead agency for scientific research on complementary and alternative medicine. Part of the National Institutes of Health.

National Center for Complementary and Integrative Health – How Safe Is This Product or Practice?

National Center for Complementary and Integrative Health – Alerts and Advisories page

National Institutes of Health – Office of Dietary Supplements

An informative website that provides information on vitamins, minerals, and other dietary supplements


National Institutes of Health: Office of Dietary Supplements. (1/4/2023). Dietary Supplements: What you need to know. Taken from

U.S. Food and Drug Association (FDA). 3/6/2023. Dietary Supplements. Taken from


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