Analytic Histamine is the name of the substance used for mind control by various agents in episode 6x19 - Three of a Kind.
It is administered by injection and nearly instantly causes the subject to become extremely suggestible, engendering in them a docile, friendly state, ideal for molding.
The mind control substance was developed by Susanne Modeski, against her will, as she grudgingly was forced to continue work on the project and held back production as long as she could. She was back working at a weapons manufacturor after being kidnapped at the conclusion of episode 5x03 - Unusual Suspects, the episode which served as an origin story for the Lone Gunmen, and also introduced them to Fox Mulder, also serving as an origin story of sorts for him.
The production of this particular mind control substance is speculative, but based in fact, as the CIA conducted tests in mind control with various substances including LSD, during the real life Project MK ULTRA.