Lorazepam, sold under the brand name Ativan among others, is a benzodiazepine medication. .. Withdrawal symptoms can occur after taking therapeutic doses of Ativan for as little as one week. Withdrawal symptoms include headaches, Onset of action: 1–5 min (IV), 15–30 min (IM). ATIVAN (Lorazepam) drug information & product resources from MPR including dosage information, educational materials, & patient assistance. May 7, - Both alprazolam (Xanax) and lorazepam (Ativan) The onset of the anti-panic effects of benzodiazepines is very rapid, beginning within the.