Clinical pharmacokinetics of lorazepam. II. Intramuscular injection

A single dose of 4 mg of lorazepam was injected into the deltoid muscles of six healthy male volunteers. Multiple venous blood samples were drawn during 48 hr after the dose and all urine was collected for 24 hr after the dose. Concentrations of lorazepam and its major metabolite, lorazepam glucuronide, were determined by electron-capture gas-liquid chromatography. Lorazepam was rapidly absorbed from the injection site, reaching peak concentrations within 3 hr. Mean pharmacokinetic pamrameters for unchanged lorazepam were: apparent absorption half-life: 21.2 min; elimination half-life: 13.6 hr; volume of distribution: 0.9 L/kg; total clearance: 58.2 ml/min. Lorazepam glucuronide rapidly appeared in plasma, reached peak concentrations within 12 hr of the dose, then was eliminated approximately in parallel with the parent drug. Within 24 hr a mean of 47.6% of the dose was recovered in the urine as lorazepam glucuronide and less than 0.5% was recovered as unchanged lorazepam.