It was a rare naturally occurring substance only found on a few planets in the Milky Way Galaxy. Given the extremely rapid and deadly symptoms of choriocytosis on species with blood not based on iron, such as Vulcans, ships working in coordination were sometimes necessary to deliver the medication to stricken individuals in time. A synthesized version of the medication could be used to slow the progress of the disease, but the recipient became immune to its effects within a few days.
When Spock contracted the virus in 2270, the USS Potemkin obtained the drug and delivered it to the USS Huron. En route to the USS Enterprise to deliver the drug, the Huron was attacked by Orion pirates, who stole the drug and the ship's other cargo, dilithium. The Enterprise eventually recovered the drug and Leonard McCoy administered it to Spock, saving his life. (TAS: "The Pirates of Orion")