Pharmacokinetics and milk residues of butorphanol in dairy cows after single intravenous administration.

The pharmacokinetics of butorphanol tartrate were investigated following intravenous administration of 0.25 mg/kg of body weight to six healthy non-lactating Jersey cows. Three lactating Holstein cows also received 0.045 mg of butorphanol/kg of body weight intravenously to determine the extent and duration of drug transfer into milk. A radioimmunoassay technique was used to measure butorphanol concentrations in plasma and milk. The disposition of butorphanol following intravenous administration was characterized by rapid and extensive distribution followed by a slower elimination phase. Apparent volume of distribution was 4.178 +/- 1.145 (mean +/- SD) 1/kg, mean elimination half-life was 82 min, and clearance was 34.6 +/- 7.7 ml/min/kg. Trace quantities of butorphanol were detected in the cow's milk for up to 36 h following administration. These pharmacokinetic data were compared with pharmacokinetic and pharmacodynamic data for butorphanol in other species and for three other potent opioids in related ruminant species.