The invention provides (i) a method of treating metabolic syndrome in an animal, (ii) a method of suppressing the appetite of an animal, (iii) a method of treating obesity in an animal, (iv) a method of reducing the weight of an animal in need thereof, (v) a method of reducing a blood lipid level in an animal in need thereof, (vi) a method of treating non-alcoholic steatohepatitis in an animal, and (vii) a method of inhibiting adipogenesis. The methods comprise administering an effective amount of an active ingredient, wherein the active ingredient comprises a diketopiperazine, a prodrug of a diketopiperazine or a pharmaceutically-acceptable salt of either of them to the animal. The invention also provides a kit comprising a container holding a diketopiperazine, a prodrug of a diketopiperazine or a pharmaceutically-acceptable salt of either of them; and instructions for administration. The diketopiperazines have the formula given in the application.