A high prevalence of adverse drug reactions (ADR) in the elderly has been explained as an outcome of multiple drug usage and slowed drug metabolism. Altered drug disposition occurs with aging but aging may not explain either the alteration in the rates of metabolism of specific drugs in older people or ADR. It is proposed that changes in the rates of drug metabolism in older people may be diet-related. Designs of human studies to separate effects of age from those of diet on drug metabolism are described. Examples are given of acute and chronic ADR that are diet or nutrition related. It is concluded that control of the diet is not only important in investigations of drug metabolism but also as a means of reducing the incidence of ADR in geriatric clinical practice.