To develop an appropriate combination of ephedrine and caffeine consisting of clinically relevant doses, we examined the acute thermogenic, metabolic, and cardiovascular effects of different doses of caffeine (C) and ephedrine (E) given separately and in combination to normal subjects. The thermogenic effect after E+C (20 mg/200mg) was larger than that of any other combinations, and E and C exerted a supra-additive synergism on thermogenesis and systolic blood pressure, while being without effect on diastolic blood pressure. The combination also had pronounced effects on glucose metabolism by increasing plasma glucose, insulin and C-peptide concentrations. During chronic treatment the effect of E+C on energy expenditure is maintained, while side effects subside because tolerance develops to its hemodynamic and metabolic effects. During dietary energy restriction E+C promotes fat loss and preserves fat-free mass, which may contribute to its chronic effect on energy balance. In conclusion, the hemodynamic and side effects to E+C are transient during chronic treatment, while the effect on energy expenditure persists. The compound also possesses repartitioning properties, which may be useful in the treatment of obesity.