Obesity is a public health problem caused by excessive consumption of high caloric diets and/or lack of physical activity. Although treatments for obesity include low caloric diets and exercise programs, these activities frequently are supplemented with appetite suppressants. For the short-term treatment of weight loss, diethylpropion (DEP) is a commonly used appetite suppressant. However, little is known with regard to how to improve its weight loss efficacy. We therefore evaluated, in rats, two administration protocols where the animals received daily injections of DEP. First, when these nocturnal animals were normally active (at night) and when they were normally inactive (daytime), and second, with or without high fat dietary restriction (HFDR). We observed that DEP induced a greater weight-loss administered when the animals were in their active phase than in their inactive phase. Moreover, DEP's administration during the inactive phase (and to a lesser degree in the active phase) promotes the consumption of food during normal sleeping time. In addition, we found that DEP-induced weight loss under ad libitum access to a HF diet, but its efficacy significantly improved under conditions of HFDR. In summary, the efficacy of DEP, and presumably other like appetite suppressants, is enhanced by carefully controlling the time it is administered and under dietary restriction of HF diets.