Lorazepam - pharmacokinetics. Onset: minutes IV, minutes IM, minutes PO. Peak: minutes IV/2 hours PO. Duration of action: hours. Lorazepam (Ativan). Learn vocabulary, terms, and more with flashcards, games, and other study tools. Pharmacological Classification: Benzodiazepines. PO: Tablet may also be given sublingually (unlabeled) for more rapid onset. Start studying Pharmacology drugs: Onset/Peak/Duration. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Lorazepam. Onset.