A harmless vitamin and mineral compound that McCoy pretends is an urgent treatment for Spock's supposed Rigelian Kassaba fever, allowing him to return to the Kelvan-held Enterprise.
Stokaline appears in the Original Series episode By Any Other Name. Trapped on a planet by the Kelvans, Spock puts himself into a deep trance so that McCoy can claim he is dying of Rigelian Kassaba fever and must be treated aboard the Enterprise.
In sickbay McCoy gives the Vulcan a shot of stokaline, telling the Kelvan Tomar that it is the only thing that will save him. When Nurse Chapel later asks about it, he admits that stokaline is nothing more than a multiple vitamin compound which will do Spock some good while changing nothing else.
The trick gets two officers back aboard the captured ship and gives them the freedom to study the Kelvans' paralysis device, the first step in the crew's plan to retake the Enterprise.