Kironide
Kironide was a substance that provided a rare source of psychokinetic abilities for humanoids when consumed. Kironide could be ingested or injected as a drug to amplify one's psionic powers. ()

History
Large deposits of kironide were present on the planet Platonius. When the Platonians, originally from , colonized the world, they gained extraordinary psychokinetic powers, which they used to entertain themselves by sadistically torturing Alexander, a Platonian who did not have the powers. It was later on discovered that they had gained these powers due to the kironide present in the local vegetation they ate, and that Alexander did not have the powers because he possessed a pituitary deficiency which prevented him from metabolizing the kironide. 

In 2268, the starship  arrived on the planet. For the landing party to escape the Platonians, Doctor McCoy injected Captain Kirk and Commander Spock with a high dose of kironide in order to reproduce the Platonians' powers. Their plan worked and they were soon able to overpower Parmen, the Platonian leader, and gain their and Alexander's freedom. ()

Appendices

Background information
According to the Star Fleet Medical Reference Manual, (pages 18, 35) kironide was an element, symbol Ki. It had the atomic weight 345 and atomic number 139. It was first discovered on . When ingested, pituitary hormones broke down the kironide compound. While kironide compounds were researched by Starfleet Medical, they were found to have no real clinical use, as the hormones had a half-life of only 10 to 30 minutes in the bloodstream.

Ron D. Moore jokingly suggested that Kironide was not used against the Dominion because the company that produced it had gone out of business. 

According to the novel The Higher Frontier, Kirk and Spock had to endure a week of nasty aftereffects from kironide poisoning and chelation therapy, thus explaining why kironide was never used again.

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Drugs