The narrator states that before the last exam, John "took a couple of bennies to pep him up, but so what? Everybody knows a couple can't do any harm." The narrator then says John should've known that barbiturates are dangerous to his nervous system. But the term "bennies" refers to Benzedrine, an amphetamine/stimulant used for staying awake. It does not refer to barbiturates, which are sedating.