Overview of Dietary Supplements

ByLaura Shane-McWhorter, PharmD, University of Utah College of Pharmacy
Reviewed/Revised Mar 2024

Dietary supplements are the most common therapies among integrative medicine and health (IMH) and complementary and alternative medicine (CAM), which include healing approaches and therapies that historically have not been included in conventional, mainstream Western medicine. Dietary supplements include medicinal herbs and nutraceuticals (products derived from food sources that claim to provide health benefits). Because the use of dietary supplements is widespread, the United States government passed the Dietary Supplement Health and Education Act (DSHEA) in 1994. It defines a dietary supplement as any product (besides tobacco) that contains a vitamin, mineral, herb or other plant product, or amino acid and that is intended as a supplement to the normal diet. Certain hormones, such as dehydroepiandrosterone (DHEA) and , also are considered dietary supplements.

The DSHEA requires that the label of a dietary supplement identify it as such. The label must state that the claims for the dietary supplement have not been evaluated by the US Food and Drug Administration (FDA). The label must also list each ingredient by name, quantity, and total weight and must identify the plant parts from which each ingredient is derived. Manufacturers are permitted to make claims about the product's structure and function (for example, good for urinary tract health), but they cannot make or imply claims for the product as a medication or therapy (for example, treats urinary tract infections). Expiration dates may be included on the standardized product labels if this information is supported by valid data demonstrating that it is not false or misleading.

Most dietary supplements used in alternative medicine are derived from plants, and some are derived from animals. Because such dietary supplements are natural, some people assume that they are safe to use. However, a substance is not necessarily safe just because it is natural. For example, many potent poisons, such as hemlock, are derived from plants, and some, such as snake venoms, are derived from animals. Furthermore, almost all substances that affect the body, whether dietary supplements or medications approved for medical use by the FDA, can have unwanted side effects.

Safety and Effectiveness of Dietary Supplements

Both prescription and nonprescription (over-the-counter) medications have been extensively and systematically studied by researchers and reviewed for safety and effectiveness by the FDA. These include studies in animals to detect the development of cancer and organ damage and studies in humans to detect any signs of toxicity. By contrast, because dietary supplements are not regulated as medications by the FDA, their manufacturers are not required to prove that supplements are safe and effective (although they must have a history of safety). Consequently, few supplements have been studied rigorously for safety and effectiveness. Furthermore, because the need to evaluate supplements in humans has been recognized only recently, much of the available information has not been gathered systematically or scientifically and so is difficult to evaluate.

Did You Know...

  • Manufacturers are not required to prove that dietary supplements are safe and effective.

The amount and quality of evidence supporting the effectiveness of supplements vary greatly. For some supplements, evidence supporting their effectiveness is convincing. However, for most, scientific studies have not been designed well enough to provide clear, reliable answers. For some supplements, the only evidence suggesting effectiveness is reports about small numbers of people or studies conducted in animals. A few supplements (for example, , chondroitin, ) are proven to be safe and useful additions to standard medications.

Evidence about the safety and effectiveness of dietary supplements is increasing rapidly as more and more clinical studies are being done. Information about such studies is available from the National Institutes of Health's National Center for Complementary and Integrative Health (NCCIH).

Additionally, manufacturers and distributors of supplements must report serious adverse events to the FDA through the FDA MedWatch system, a medical product safety reporting program.

Purity and Standardization of Dietary Supplements

Other areas of concern are the purity and standardization of supplements. Supplements, unlike medications, are not regulated to ensure that they are pure or that they contain the ingredients or the amount of active ingredient they claim to contain. As a result, the supplement may contain other substances that may be inert or harmful, including, in some cases, prescription or nonprescription medications, natural toxins, bacteria, pesticides, unapproved dyes, and even dangerous substances such as heavy metals, including lead and mercury.

The amount of active ingredient in a dose of a supplement may vary, especially when whole herbs are ground or made into extracts to produce a tablet, capsule, or solution. The buyer is at risk of getting less, more, or, in some cases, none of the active ingredient in a supplement. Standardization requires that each individual dosage form of the product contain a precise amount of its active ingredient or ingredients. However, most herbal products are mixtures of several substances, and which ingredient is the most active is not always known. Therefore, determining which ingredient or ingredients should be considered active and thus subject to standardization can be difficult. Some supplements, particularly those produced in Europe, have been standardized and may include a designation of standardization on the label.

Advice on how to choose a pure, standardized product varies from expert to expert. Most experts recommend buying from a well-known manufacturer, and many recommend buying products made in Germany because oversight of supplements is stricter there than in the United States.

Although the content of a supplement is not standardized, the way in which it is manufactured has been standardized. In 2007, the Food and Drug Administration (FDA) created current good manufacturing practices (GMPs) that standardize the manufacturing, packaging, labeling, and storing of dietary supplements. These GMPs help ensure the quality of dietary supplements and help protect the public health.

Drug Interactions with Dietary Supplements

Supplements can interact with prescription and nonprescription drugs. Such interactions may intensify or reduce the effectiveness of a medication or cause a serious side effect. Before taking supplements, people should consult their doctor, so that such interactions can be avoided. Few well-designed studies have been conducted to investigate supplement-drug interactions, so most information about these interactions comes from sporadic individual reports of interactions.

Other Concerns of Dietary Supplements

In addition to drug interactions, there are other potential issues with using dietary supplements:

  • Supplements (especially herbal products) may not be stable once manufactured and may, as a result, yield inconsistent benefits or no benefit at all.

  • Unlike with prescription medications, there is often little evidence about the best dose of supplements.

  • People may use dietary supplements instead of conventional medications prescribed by their doctor.

  • Supplements may be toxic and cause harm.

  • Supplement use may contribute to incorrect diagnosis of a medical problem because placebo effects can simulate true benefits, particularly if the person and/or doctor strongly believe in the supplement. Therapeutic responses to supplements, placebo-mediated or otherwise, could be mistaken as evidence that confirms a particular, possibly incorrect, diagnosis.

  • Young children and people who are pregnant should be particularly careful with supplements, perhaps using them only in unusual situations.

To avoid or manage these potential issues, it is important that people disclose their use of all dietary supplements to their doctor.

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