Pharmacokinetics of naloxone and naltrexone in the dog

The serum kinetics of 5 mg/kg of i. v. naloxone or naltrexone were studied in each of two groups of five dogs; serum samples were obtained from 2 min to 2 hr after injection. Serum concentrations were determined by radioimmunoassay. Serum levels of both naloxone and naltrexone fell rapidly; serum half-life during the elimination phase was 71.2 +/- 8.9 min (mean +/- S.E.) for naloxone and 85.1 +/- 9.0 min (mean +/- S.E.) for naltrexone. Although human and dog kinetics are similar for naloxone, naltrexone is long-acting in man, but is quickly dissipated in the dog.