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Dietary supplements may contain harmful or undesirable substances such as pesticides or heavy metals. Contaminants may be present before a substance is harvested for use as a dietary supplement or may be introduced during processing and packaging.
For example, coral calcium, a dietary supplement said to contain remnants of living coral reefs, may contain significant and harmful amounts of lead and other heavy metals.
Consumption of excessive levels of lead can cause neurological problems, increased blood pressure, reproductive impairment, and hearing and sight problems. Scientists are also concerned that people who are allergic to shellfish may experience serious adverse events such as hives, swelling, and breathing problems, because of the presence of these allergens in coral reefs. In 2003 the Sports Nutrition Working Group of the International Olympic Committee Medical Commission
reported that approximately one in five dietary supplements commonly used by athletes were contaminated. Protein powders; amino acid supplements; creatine; pyruvate; and several vitamin, mineral, and herbal supplements were found to contain steroid-like chemicals that were not identified on product labels, caused positive doping tests, and were not known to be safe.
Dietary supplements may also contain controlled substances. Acetaminophen, aspirin, antihistamines, and corticosteroids have been found in dietary supplements that did not list them as ingredients. In 2005 the FDA issued a warning about a dietary supplement that contains the antidiabetic agent glyburide after it caused hypoglycemia (dangerously low blood sugar) in several consumers. In 2006 the FDA warned consumers against using several weight-loss dietary supplements that contain chlordiazepoxide HCl (the active ingredient in the drug Librium) and fluoxetine HCl (the active ingredient in the antidepressant Prozac).

Dietary supplements are natural substances that may contain variable amounts of active and inactive ingredients. These amounts depend on several factors, including the age and specific part of the plant, the composition of the soil in which it was grown, the time of year it was harvested, and how the substance was processed and stored. While manufacturers must label their products with the contents, the recommended daily dose, and how much of the active ingredient each dose contains, dietary supplement composition can vary widely. From bottle to bottle, batch to batch, year to year, and company to company, consumers may not be aware of exactly what they are consuming.
In a 2003 study published in Archives of Internal Medicine, researchers analyzed 880 herbal dietary supplements collected from a variety of sources, including grocery stores, retail pharmacies, discount stores, and health food stores. They compared the dietary supplement ingredients and the recommended daily doses stated on the label with their own laboratory analyses. Of the products sampled, 43 percent were consistent with a standardized pharmacy reference for ingredients and recommended daily doses, 20 percent were consistent regarding ingredients only, and 37 percent were either inconsistent or did not contain enough information on the label to determine consistency. The authors suggested that much of this inconsistency could be attributed to insufficient research on the part of the manufacturer and inadequate understanding of how to ensure consistency in a natural resource.
A study presented at the Sixth International Cartilage Repair Society Symposium in 2006 analyzed ten commercially available dietary supplements containing chondroitin sulfate, which is commonly used to improve joint pain associated with osteoarthritis. Researchers evaluated the ability of the raw materials to inhibit gene expression of three substances involved in cartilage breakdown. These results were compared with a reference standard, which was the chondroitin sulfate supplement used in a previous study, the largest federally funded trial evaluating the supplement’s clinical effects. Only one of the ten supplements inhibited gene expression in a manner equivalent to the reference standard. Two of the supplements had no detectable effect, and the remainder had inconsistent effects on gene expression.