Pharmacokinetics of temazepam after day-time and night-time oral administration

The pharmacokinetic disposition of temazepam was compared after a day-time and night-time dose in an open randomised crossover study. Twelve healthy male volunteers received a single oral dose of 20 mg temazepam in a soft gelatine capsule at 0900 h or 2200 h. Blood samples were taken immediately before dosing and at selected times over the 36-h period after each dose. The absorption of temazepam was slower after evening administration; the absorption half-life and time to reach maximal plasma concentration being 0.53 h and 1.67 h respectively, compared to 0.38 h and 1.02 h following morning administration. Considering distribution characteristics, evening administration produced a lower peak plasma temazepam concentration (362 ng/ml) compared with a day-time level of 510 ng/ml. Distribution half-life after night-time administration was increased compared with day-time administration (1.76 h vs 1.03 h). A significantly higher percentage of the drug, relative to Cmax, remained in the plasma at 8 and 24 h after evening dosing (39.3 and 15.4% compared to 24.7 and 11.2% following day-time administration). In spite of the half-lives of absorption, distribution and elimination all being longer after the evening dose, the overall bioavailability, as measured by the area under the curve (AUC) was comparable after the two times of administration. Similarly the difference in the mean residence time (MRT) of the two doses was within accepted limits. It is concluded that a chronopharmacokinetic effect was seen for temazepam; however it is unlikely to be of any clinical significance.