What quantity of 0.5mg ativan (lorazepam) is considered a dangerously high dose?

Depends on the. Person. If you are not used to taking ativan (lorazepam), (lorazepam) then you can become overly sedated with just a total of 2-3 mgs. If you regularly take Ativan (lorazepam) and have built up a tolerance, then you could take much more. When prescribing, most physicians follow the idea of "start low and go slow" when dealing with medications like ativan (lorazepam).