Pharmacokinetics of pethidine administered intramuscularly and intravenously to dogs over 10 years old.

The pharmacokinetics of pethidine administered postoperatively both intravenously and intramuscularly was investigated in dogs aged over 10 years. When the drug was given intravenously (2 mg kg-1), plasma levels declined in a biexponential manner, with an elimination half-life of 62.7 minutes. Following its intramuscular administration at the same dose rate, absorption was very slow and two distinct rates of absorption were observed. Maximum plasma concentrations were not achieved until 33 minutes after drug administration. The elimination of the intramuscularly administered drug was also slow with a t 1/2 beta' of 145.9 minutes. Thus, it seems that in elderly animals pethidine has a long elimination half-life and a slowed rate of absorption. However, the total body clearance of the drug does not seem to be affected by age.