Variations in the bioavailability of thiazinamium methylsulfate

Bioavailability after oral administration of the anticholinergic drug thiazinamium methylsulfate (Multergan), a phenothiazine derivative with a quaternary ammonium group in the molecule, has been studied in patients and volunteers by measuring the drug concentrations in plasma or the excretion of the parent drug in urine. The relative bioavailability as compared to intramuscular injection seems to be of the order of 10%. Much more of the drug is absorbed, however, but is metabolized during the first liver passage. Moreover, there seems to be a substantial interindividual variation in the bioavailability of the drug. Studies in a group of eight volunteers showed that there is also a substantial intraindividual variation, but its magnitude is smaller than that of the interindividual variation.