Underlying most theoretical statements about the action of weight reducing drugs is the implicit belief that obesity is a resultant effect of calorie intake exceeding calorie expenditure. Consequently, the task of an anti-obesity drug may be designated as the reduction of energy intake or as the increase in rate of use of energy by the body. Although thyroid hormones have been used to produce weight loss by enhancing energy expenditure, this general approach to the chemical treatment of obesity has not been favoured in research or in practice. Instead, the development of drugs useful for obesity therapy has largely focussed upon the capacity of drugs to produce anorexia. The anorexic potential of such drugs is first evaluated in pharmacological screening tests which monitor the food consumption of animals over specified intervals of time before human clinical trials are conducted, during which the efficacy of a drug is measured in terms of pounds of weight lost per week.