Simultaneous analysis of codeine and its active metabolites in human plasma using liquid chromatography-tandem mass spectrometry: Application to a pharmacokinetic study after oral administration of codeine

► The assay simultaneously determines codeine and its active metabolites in human plasma. ► The assay is at least ten times more sensitive than any previously published methods. ► The assay needs a short 5 min run time, which was of > two times shorter than previous reports. ► This is the first pharmacokinetic study of codeine in Mongolian Chinese population.