Pharmacokinetics of subcutaneous and intramuscular butorphanol in dogs

Butorphanol tartrate was administered intramuscularly and subcutaneously to adult male and female dogs at a dose of 0.25 mg/kg. No significant absorption lag time and no significant difference bwtween peak intramuscular and subcutaneous serum concentrations were observed. The mean peak serum concentration was 29 ng/ml at mean times of 28 min after subcutaneous administration and 40 min after intramuscular administration. There were no significant differences in the pharmacokinetics of butorphanol in dogs with either route. The serum half-life was 1.62 hr, and the serum clearance was 3.45 liters/kg/hr. The apparent volume of distribution of butorphanol was 7.96 liters/kg. Although considerable inter- and intraanimal variation in Cmax and AUC was observed, there was no significant difference in the area under the serum concentration versus time curves, and the two administration routes were considered bioequivalent.