Psycho

 Psycho appears to be 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 Constantine Chase and was supposed to increase the combat potential of a soldier, whilst dampening higher mental functions, making for fearless, but uncontrollable troops. 

Fallout, Fallout 2 and Fallout Tactics
Interestingly, psycho has another effect not intended by the programmers. When taken, it reduces Intelligence by 3. This can be exploited to boost your skill points. First, raise a skill to the point it needs two skill points to be increased, then level up. When you level up, take two to reduce your Intelligence greatly. This will correspondingly lower your skills involving Intelligence (science, repair, outdoorsman, doctor, etc.). Now allocate your skill points to the skill you want raised - since the skill's value has been lowered, it will take only one skill point to do so. Then rest for a few hours. When the psycho wears off, Intelligence will be restored, and the skill will be given a significant boost. This is particularly useful for getting a 121% science skill to recruit Skynet without taking Mentats or wasting too many skill points.

Fallout 3
Psycho is very common in the Capital Wasteland. It is sometimes sold secretly, requiring dialogue and possibly a speech check to convince the proprietor. There are around 60 in set locations in-game.

In the Operation: Anchorage add-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.

Locations

 * At least one dose can be found in Silver's house.
 * Several doses can be found in the Super Duper Mart.
 * Frequently found in raider establishments.
 * Sold by Doc Hoff, Cindy Cantelli, Ahzrukhal, Leo Stahl, and various other vendors.
 * Three can be stolen from A Quick Fix in Rivet City.
 * Four can be stolen from Cantelli's room in Rivet City.
 * Dead bodies of wastelanders and feral ghouls.
 * Can be found on the corpses of Talon Company merc and raiders.
 * Sold by Wasteland Junkie around burning barrels in the wastelands, along with jet, Mentats, and other addictive chems. (See Fallout 3 random encounters)
 * Given by field doctor during Operation: Anchorage (add-on).

Fallout: New Vegas
The player may consume psycho or use it as an ingredient in slasher, which increases both damage and damage resistance but has a much shorter duration.

Locations

 * In the Dino Dee-lite motel, second room from the left on the top floor.
 * 2 on the floor of the Great Khans hideout in Boulder City, behind the shop counter.
 * Can sometimes be found on Fiends and ghouls.
 * Several can be found on the ground at the Tribal Village
 * Can be found in and around the Bison Steve hotel.
 * Inside most of the rooms in the Jacobstown Lodge.
 * Is sold by Jack at Red Rock Drug Lab.

Related quests

 * High Times: Jacob needs 1 dose for the improved cure the player can suggest with 50 science.
 * Flags of Our Foul-Ups: Razz wants you to convince his squad to use it.
 * Back in Your Own Backyard: Ranger Milo can be impressed by the Courier's possession of 24 or more doses of the drug, earning a small amount of NCR fame.
 * Aba Daba Honeymoon: After delivering the Khans drug shipment to Motor-Runner, the player can sell the drug to him in exchange for 20 caps, 22 after a barter check. The most the player can ever sell the drug for normally is 20 caps with high barter or discounts.
 * Malleable Mini Boomer Minds: The player can offer one of the Boomer children the drug by telling him it will make him a better boxer, this results in Boomer infamy.
 * Happy Trails Expedition: If the player can pass a medicine check of 45 when speaking to Ricky, then they can sell him 1-10 hits of the drug for the exorbitant price of 30 caps each. This is the most money the player can get for the drug in the game.

All Roads
The drug is featured prominently in the deaths of the Singer (poisoned by Swank), Chance (overdose enabled by Benny) and a group of Fiends that burned their victims at the Tribal Village. Benny nicknames the drug during one line, "Only thing in that vial is Vegas".

Behind the scenes
While it may not contain similar drugs, psycho's background is similar to that of the real-life drug cocktail D-IX, developed by Nazi Germany to increase military effectiveness.