Talking to the doctor in Megaton can get him to reveal Leo's crazy chem addiction, namely to jet. After this you can talk to him and talk him into quitting (good Karma). He will give you a key. Go to the Water processing plant, and there is a locked desk full of jet and money (along with some other chems).
Talk to Vera Weatherly and ask her about some gossip. She can tell you some interesting stuff. Mister Lopez is considering suicide, for example. She can also tell you that Paulie Cantelli is addicted to jet. Asking around confirms this. His room is filled with Jet, Psycho, and other chems. It is not free to take and you will lose Karma for doing so. However, you can avoid getting caught by simply closing the door behind you. While speaking to him, you have the option of offering him psycho. If you give him enough psycho you will soon find him dead (see Overdose (quest)).
When talking to him, he tells you he wants to cut your hair. Then a dialog option is available: "Are you taking jet over here?", to this he replies he has nothing else to do but get high. This is strange, considering how Murphy says that Jet has little effect on ghouls.
Chem addicts who have gone insane with their chems and are hostile to the player can be found predominately around the El Rey Motel. One is found in one of the rooms of the second level of the El Rey Motel. The others in the El Rey are dead, often lying on the floor.
Two non-hostile chem addicts are found at a camp fire just south of Fields' shack across the railroad tracks.