A dose-ranging study of the pharmacokinetics of codeine phosphate following intravenous administration to rats

The linearity of the pharmacokinetics of codeine was examined in male Sprague-Dawley rats given iv bolus doses of 1, 1.5, 3, and 4 mg/kg of codeine phosphate. Codeine and morphine were determined in serial blood samples utilizing HPLC with electrochemical detection. Codeine exhibits characteristics consistent with a two-compartment pharmacokinetic model. The kinetics of codeine are linear in the iv dose range 1-4 mg/kg. The ratio AUCmorphine:AUCcodeine increases disproportionately with increasing doses of codeine.