Oct 10, - ACTION. Ativan (lorazepam) is an active benzodiazepine with a peak plasma levels are reached within minutes, whereas following. Lorazepam serum levels are proportional to the dose administered. Giving 2 mg oral lorazepam will result in a peak total serum level of around 20 ng/ml around two hours later, half of which is lorazepam, half its inactive metabolite, medicalcases.eu of action: 1–5 min (IV), 15–30 min (IM). Benzodiazepine, Onset of Action2, Peak Onset (hrs), Half-life parent (hrs) Clonazepam1 (Rivotril®), Int. , , -, mg. Lorazepam1 (Ativan®). Int. (po).