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Overview of Dietary Supplements

By

Laura Shane-McWhorter

, PharmD, University of Utah College of Pharmacy

Last full review/revision Jul 2020| Content last modified Jul 2020
Click here for the Professional Version
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Topic Resources

Integrative medicine and health (IMH) and complementary and alternative medicine (CAM) include healing approaches and therapies that historically have not been included in conventional, mainstream Western medicine. The most commonly used IMH and CAM therapies involve using dietary supplements, which include medicinal herbs and nutraceuticals. 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 melatonin, also are considered dietary supplements.

The act 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 drug or therapy (for example, treats urinary tract infections). Expiration dates are often included on the standardized product labels.

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 drugs approved for medical use by the FDA, can have unwanted side effects.

Safety and Effectiveness

Because dietary supplements are not regulated as drugs 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. Manufacturers must now report serious adverse events to the FDA through the FDA MedWatch system. However, a few supplements (for example, fish oil, chondroitin, glucosamine, St. John's wort) are proven to be safe and useful additions to standard drugs.

Did You Know...

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

In contrast, both prescription and nonprescription (over-the-counter) drugs 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.

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 individual people or studies conducted in animals.

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).

Purity and Standardization

Other areas of concern are the purity and standardization of supplements. Supplements, unlike drugs, 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 drugs, 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.

Interactions With Drugs

Supplements can interact with prescription and nonprescription drugs. Such interactions may intensify or reduce the effectiveness of a drug 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.

Table
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Some Possible Dietary Supplement–Drug Interactions

Medicinal Herb

Affected Drugs

Interaction

Anticoagulants (drugs that prevent blood clots, such as warfarin)

Chamomile taken with anticoagulants may increase the risk of bleeding.

Sedatives (such as barbiturates and benzodiazepines)

Chamomile may intensify or prolong the effects of sedatives.

Chamomile may reduce iron absorption.

Drugs that can damage the liver (such as amiodarone, anabolic steroids, ketoconazole, and methotrexate)

Echinacea taken for more than 8 weeks may damage the liver. When echinacea is taken with another drug that can damage the liver, the risk of liver damage may be increased.

Immunosuppressants (drugs that intentionally suppress the immune system, such as corticosteroids and cyclosporine)

By stimulating the immune system, echinacea may negate the effects of immunosuppressants.

Ephedra*

Stimulant drugs (such as caffeine, epinephrine, phenylpropanolamine, and pseudoephedrine)

Ephedra contains ephedrine, which is a stimulant that increases the stimulant effects of other drugs, increasing the risk of irregular or rapid heart rate and high blood pressure.

Monoamine oxidase inhibitors (MAOIs, a type of antidepressant)

Ephedrine may intensify the effects of these drugs and increase the risk of side effects, such as headache, tremors, irregular or rapid heart rate, and high blood pressure.

Anticoagulants (such as warfarin)

Feverfew taken with anticoagulants may increase the risk of bleeding.

Feverfew may reduce iron absorption.

Feverfew may increase heart rate and blood pressure when it is taken with drugs used to manage migraine headaches.

NSAIDs reduce the effectiveness of feverfew in preventing and managing migraine headaches.

Anticoagulants (such as warfarin)

Garlic taken with anticoagulants may increase the risk of bleeding.

Antiplatelet drugs (such as aspirin or clopidogrel)

Garlic may increase risk of bleeding.

Antihypertensives

Garlic may increase antihypertensive and BP-lowering effects.

Isoniazid

Garlic may reduce absorption of isoniazid.

Protease inhibitors (such as indinavir or saquinavir), which are used to treat human immunodeficiency virus (HIV) infection

Garlic decreases blood levels of protease inhibitors, making them less effective.

Anticoagulants (such as warfarin)

Ginger taken with anticoagulants may increase the risk of bleeding.

Antiplatelet drugs (such as aspirin or clopidogrel)

Ginger may increase risk of bleeding.

Anticoagulants (such as warfarin), and antiplatelet drugs (such as aspirin, and other NSAIDs)

Ginkgo taken with anticoagulants or with aspirin or other NSAIDs may increase the risk of bleeding.

Anticonvulsants (such as phenytoin)

Ginkgo may reduce the effectiveness of anticonvulsants in preventing seizures.

Monoamine oxidase inhibitors (MAOIs, a type of antidepressant)

Ginkgo may intensify the effects of these drugs and increase the risk of side effects, such as headache, tremors, and manic episodes.

Anticoagulants (such as warfarin), and antiplatelet drugs (such as aspirin, and other NSAIDs)

Ginseng taken with anticoagulants or with aspirin or other NSAIDs may increase the risk of bleeding.

Ginseng may intensify the effects of these drugs, causing an excessive decrease in blood sugar levels (hypoglycemia).

Ginseng may intensify the side effects of corticosteroids.

Digoxin

Ginseng may increase digoxin levels.

Estrogen therapy

Ginseng may intensify the side effects of estrogen.

Ginseng can cause headache, tremors, and manic episodes when it is taken with MAOIs.

Opioids (narcotics)

Ginseng may reduce the effectiveness of opioids.

Anticoagulants (such as warfarin)

Goldenseal may increase the effects of anticoagulants and may increase the risk of bleeding.

Antihypertensives

Berberine content may increase antihypertensive effects, possibly decreasing blood pressure too much.

Antihyperglycemics

Berberine may increase hypoglycemic effects

Warfarin

Green tea may cause warfarin to be less effective.

Sedatives (such as barbiturates and benzodiazepines)

Kava may intensify or prolong the effects of sedatives.

Licorice (Glycyrrhiza glabra)†

Licorice may increase salt and water retention and increase blood pressure, making antihypertensives less effective.

Licorice may increase the risk of an abnormal heart rhythm, making antiarrhythmic therapy less effective.

Digoxin

Because licorice increases urine formation, it can result in low levels of potassium, which is excreted in urine. When licorice is taken with digoxin, the low potassium levels increase the risk of digoxin toxicity.

Licorice may intensify the effects of most diuretics, causing increased, rapid loss of potassium. Licorice may interfere with the effectiveness of potassium-sparing diuretics, such as spironolactone, making these diuretics less effective.

Licorice may intensify the effects of these drugs and increase the risk of side effects, such as headache, tremors, and manic episodes.

Milk thistle may intensify the effects of these drugs, causing an excessive decrease in blood sugar levels.

Protease inhibitors (such as indinavir or saquinavir), which are used to treat HIV infection

Milk thistle decreases blood levels of protease inhibitors, making them less effective.

Warfarin

Milk thistle may increase risk of bleeding by increasing effects.

Estrogen therapy and oral contraceptives

Saw palmetto may affect hormone levels.

Antiplatelets and anticoagulants 

Saw palmetto may increase effects and may cause bleeding.

Cyclosporine

St. John’s wort may reduce blood levels of cyclosporine, making it less effective, with potentially dangerous results (such as rejection of an organ transplant).

Digoxin

St. John’s wort may reduce blood levels of digoxin, making it less effective, with potentially dangerous results.

Iron

St. John’s wort may reduce iron absorption.

St. John’s wort may intensify the effects of MAOIs, possibly causing very high blood pressure that requires emergency treatment.

St. John’s wort increases the metabolism of these drugs, reducing their effectiveness.

St. John’s wort increases the metabolism of these drugs, reducing their effectiveness.

Photosensitizing drugs (such as lansoprazole, omeprazole, piroxicam, and sulfonamide antibiotics)

When taken with these drugs, St. John’s wort may increase the risk of sun sensitivity.

Protease inhibitors (such as indinavir or saquinavir), which are used to treat HIV infection

St. John’s wort may reduce blood levels of protease inhibitors, making them less effective.

Selective serotonin reuptake inhibitors (SSRIs, such as fluoxetine, paroxetine, and sertraline)

St. John’s wort may intensify the effects of these drugs.

Tricyclic antidepressants

St. John's wart may decrease the effects of these drugs.

Warfarin

St. John’s wort may reduce blood levels of warfarin, making it less effective and clot formation more likely.

Sedatives (such as barbiturates and benzodiazepines)

Valerian may intensify or prolong the effects of sedatives, causing excessive sedation.

* Sale of supplements containing ephedra is banned in the United States.

† True, natural licorice, not the more common, artificially flavored licorice candy.

Other Concerns

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.

  • People may use dietary supplements instead of conventional drugs 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.

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

More Information

The following are some English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

Drugs Mentioned In This Article

Generic Name Select Brand Names
AFRINOL, SUDAFED
ALDACTONE
NIZORAL
PREVACID
NEORAL, SANDIMMUNE
OTREXUP
ADRENALIN
PLAVIX
ZOLOFT
INVIRASE
ERGOMAR
CORDARONE
PRILOSEC
PAXIL
PROZAC, SARAFEM
LANIAZID
CRIXIVAN
DILANTIN
FELDENE
COUMADIN
No US brand name
LANOXIN
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
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