Ativan (lorazepam) usually works within half an hour to an hour if the dose is sufficient. How long is ativan detectable by drug tests? Pin On All In A Days Work Using ativan for more than 4 months in a row hasn’t been studied. How long is ativan working . However, peak concentrations in the blood occur after about two hours. Summary of fda black box warnings Lorazepam peak concentrations are reached at about two hours. Valium starts working within about 15 minutes. After an oral dose, lorazepam will start to work within an hour. Ativan (lorazepam) is considered a rapid acting benzodiazepine medication, with an intermediate duration of action. A us doctor answered learn more. Symptoms of insomnia or anxiety should improve quickly after a dose. How long does it take for 1 mg ativan to work? The full sedating effect lasts for around 6 to 8 hours. This means that the concentration of lorazepam in your body is reduced by half every 12 hours. About 15 to 30 mi...