Pax

The drug used to alter the colonists on Miranda was called "Pax" or G-23 Paxilon Hydrochlorate. We know very little about it, other than it was spread by air from the atmosphere processors. But we can deduce a couple things: One, it must have had a very low required dosage, considering it was spread into the air from the atmosphere processors like pollution from a factory. Two, it was odorless, tasteless, etc, although that may have been part of its low concentration. And three, the effects are permanent; thirteen years later, the reavers are still insane.

When deciding about clues in the atmosphere processor (Secrets of Miranda), I looked up real psychoactive drugs to see what elements are in them, besides the expected Carbon, Nitrogen, Hydrogen and Oxygen. Chlorine and Fluorine are common, and since those elements are dangerous to breathe, there would be no reason an atmosphere processor would be adding them to a breathable mix. So they became the mystery elements at the atmosphere processor.