A drug prescribed by Sarek's physician for a malfunctioning heart valve, which the ambassador was taking in secret when he boarded the Enterprise for the Babel conference.
Benjisidrine is mentioned in the Original Series episode Journey to Babel. Ambassador Sarek has suffered several cardiac attacks before the voyage and has told neither his wife Amanda nor his estranged son Spock; his doctor on Vulcan prescribed benjisidrine for the condition.
Sarek reveals the medication only after he collapses aboard the Enterprise, which also explains where he was at the time the Tellarite ambassador Gav was murdered. The drug has not been enough, and McCoy concludes that only open-heart surgery will save him.
The operation needs large quantities of the rare T-negative blood type, forcing Spock to act as donor and setting up the episode's conflict between his duty to the ship and to his father.