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.
Depending on the game, Psycho will give different effects. For Fallout 1 and 2, Psycho will increase damage resistance, allowing subjects to survive hits more easily. For Fallout 3 and Fallout: New Vegas, Psycho will increase the damage the dealt with weapons. In the latter game, the Psycho drug is an ingredient for the Slasher drug.
Slasher is a chem which combines the effects of Med-X and Psycho into a single drug. It is possible to combine the effects of this drug with Med-X to gain a 50% boost to damage resistance. It will not stack with Psycho.