Is it ensure, insure, or assure?
1700, "an excessive dose," from over- + dose (n.).
1727, "to administer medicine in too large a dose" (transitive); from 1968 as "to take an overdose of drugs;" see over- + dose (v.). Related: Overdosed; overdosing.
overdose o·ver·dose (ō'vər-dōs')n. An excessive dose, especially of a narcotic.