A four-year evaluation of the chronic toxicity of megestrol acetate in dogs.

A 4-year evaluation of the chronic toxicity of megestrol acetate in dogs is reported. .01, .1 or .25 mg of megestrol acetate/kg/day or .25 mg of chlormadinone acetate/kg/day was administered orally for 4 years t o female beagle dogs. The hormone-treated dogs tended to gain more weig ht than did the controls (controls vs. .25 mg megestrol acetate every month after the 3rd p less than .01). All treated dogs revealed decreased evidence of estrus. Mucoid vaginal discharges were more prevalent among the middle and high dose groups. Mean hemoglobin, packed cell volume and total erythrocyte values were slightly decreased while mean total leucocyte count and erythrocyte sedimentation rates were slightly increased in the middle and high dose groups. Clotting me chanism did not reveal any disturbances. Evidence of diabetes consistin g of bilateral cataracts, elevated serum glucose concentrations and glycosuria after 4 years in 2 of 16 high-dose megestrol acetate and in 6 of 15 chlormadinone acetate-treated dogs was revealed. It is concluded that the effects of megestrol acetate were similar but less severe than those of chlormadinone acetate.