Pharmacokinetics and relative bioavailability of levomepromazine after repeated administration of tablets and syrup

Plasma levels of levomepromazine and its sulphoxide were measured in 8 psychiatric patients after repeated doses of levomepromazine tablets or syrup. The rate and extent of absorption of the drug were similar for the two dosage forms, although the extent of presystemic metabolism appeared to be slightly greater after administration of syrup than of tablets. The biological half-life of levomepromazine ranged from 16.5 h to 77.8 h, and a 13-fold variation was seen in the ratio of the total clearance to the absorbed fraction of the dose (Cl/Fpo). It is postulated that individual variation in the dose required for therapy was due in part to individual variation in the pharmacokinetics of the drug.