With Ativan overdose dosages in mind, it's easier to understand the medication's LD50. This value refers to the lethal dosage in 50 percent of mice laboratory tests. The amount comes in at 1,850 mg/kg for Ativan. Of course, this is only a general rule of thumb and provides only a theoretical correlation with humans. Using this number as a baseline, we can determine that several thousand milligrams are required to produce an immediate fatal overdose of Ativan.