Psycho appears as a form of injected drug composed of strange, unknown chemicals of military origin, that comes with its own unique delivery system. It was designed at the orders of General Chase specifically for military use. It increases a soldier's damage resistance, and its dampening effect on higher brain functions make for tough, but uncontrollable, troops. This drug was very popular for human-wave assaults.
In the Operation: Anchorageadd-on, the origins of the chem are explored in more detail. The American field doctor in the simulation can give the player psycho before combat, and a holodisk can be found that details the creation and use of the drug. It is also revealed that it can cause anger issues and other dangerous, long-lasting side effects.