Foods rich in vitamins, especially those containing beta carotene and vitamin C, lower the risk of cancer in humans. Whether vitamins alone are responsible for this reduced risk is not known. They may work synergistically with increased dietary fiber and decreased fat intake. Vitamin supplementation at doses higher than the recommended daily allowance is, at present, an unknown factor in preventing cancer and may cause toxic effects in some instances. Consuming megadoses doses of any vitamin or mineral as a cancer preventive should be discouraged. Physicians need to take a more aggressive stance against alternative health care providers who recommend questionable dietary or nutritional regimens. Because of the proliferation of food quacks in recent years and the danger they represent to the general public, physicians need to make preventive health care an important part of their responsibility. This includes educating patients on proper nutrition.