Over-the-counter dietary supplements to treat obesity appeal to many patients who desire a "magic bullet" for weight loss. Asking overweight patients about their use of weight-loss supplements and understanding the evidence for the efficacy, safety, and quality of these supplements are critical when counseling patients regarding weight loss. A schema for whether physicians should recommend, caution, or discourage use of a particular weight-loss supplement is presented in this article. More than 50 individual dietary supplements and more than 125 commercial combination products are available for weight loss. Currently, no weight-loss supplements meet criteria for recommended use. Although evidence of modest weight loss secondary to ephedra-caffeine ingestion exists, potentially serious adverse effects have led the U.S. Food and Drug Administration to ban the sale of these products. Chromium is a popular weight-loss supplement, but its efficacy and long-term safety are uncertain. Guar gum and chitosan appear to be ineffective; therefore, use of these products should be discouraged. Because of insufficient or conflicting evidence regarding the efficacy of conjugated linoleic acid, ginseng, glucomannan, green tea, hydroxycitric acid, L-carnitine, psyllium, pyruvate, and St. John's wort in weight loss, physicians should caution patients about the use of these supplements and closely monitor those who choose to use these products.