A naturally occurring drug, the only known cure for choriocytosis, a disease that is a passing nuisance to humans but fatal within days to Vulcans.
Strobolin is the object of the chase in the episode The Pirates of Orion. An outbreak of choriocytosis aboard the Enterprise seems to be over when Spock collapses. In copper-based Vulcan blood the infection stops cells carrying oxygen, and McCoy gives him three days.
The nearest supply is too far for the Enterprise to reach in time, so the freighter SS Huron is sent to meet the starship halfway with the drug.
Orion pirates attack the Huron and steal its cargo, including the strobolin. Kirk tracks them to an asteroid field and confronts their captain, who would rather destroy both ships than let Orion's supposed neutrality be exposed. Kirk prevents the suicide and the drug reaches Spock with little time to spare.