In the US, the FDA advises against use of benzodiazepines such as lorazepam for longer than four weeks. It is fast acting, and useful in treating fast onset panic anxiety. Withdrawal symptoms, including rebound insomnia and rebound anxiety, may occur after seven days' use of medicalcases.eu of action: 1–5 min (IV), 15–30 min (IM). Mar 10, - Learn about Ativan (Lorazepam) may treat, uses, dosage, side effects, a more rapid onset or prolonged effect of lorazepam due to increased half-life can appear following cessation of recommended doses after as little as. Oct 10, - After sublingual administration, peak plasma levels occur at 60 minutes. By the intramuscular route, the absorption half-life values of lorazepam average 12 and 19 minutes, whereas by the oral route, there is an additional lag period averaging 15 and 17 minutes.