Benzodiazepine, Onset of Action2, Peak Onset (hrs), Half-life parent (hrs) (Ativan®). Int. (po), Rapid (sl, IV). (po), , -, 1mg. Oxazepam1 (Serax®). Ativan/Lorazepam Intravenous Inj Sol: 1mL, 2mg, 4mg Buspirone has a slow onset of action and the drug will not block the withdrawal syndrome often seen. Mar 10, - Learn about Ativan (Lorazepam) may treat, uses, dosage, side effects, in a more rapid onset or prolonged effect of lorazepam due to increased . However, in one study involving single intravenous doses of to 3 mg of.