Leo's Drug Habit

Stahl's addiction refers to a mini-quest in Fallout 3.

Obtaining the Quest
By either talking to Doc Church or Mr. Burke one can open up a dialog option with Leo Stahl to talk about his Chem habit. If the player character asks Doc Church about the people of Megaton, then uses the Speech or the Medicine dialog option, he'll tell you about Leo Stahl's addiction to Jet.

Alternatively in the quest The Power of the Atom you can agree to blow up Megaton for Mr. Burke (though you don't really have to blow it up). If the player character then expresses to him concern that he/she can't do it, Mr. Burke will tell you about Leo Stahl's chem habit.

Good Karmic Path
Go talk to Leo Stahl in the The Brass Lantern during the day or the Water Processing Plant at night and ask him about his Chem habit. The good Karma way is to use the Speech dialog option to (if you're successful and you only have one chance) quit his addiction, he'll then ask you to clean up his stash in the Water Processing Plant. You'll get 100xp (unmodified), good Karma and his key to a stash of Chems and Caps in the desk at the Water Processing Plant.

Bad Karmic Path
Alternatively the bad Karma way is if the player character asks him "What's it worth to you to keep me quiet?" he will start dealing chems to the PC, and the PC will receive bad Karma. Later, you can kill him (if you do it where nobody can see you then nobody'll go hostile) and loot the key to his stash from his corpse; you'll obtain more bad Karma for killing him.

Upon taking the Bad Karmic Patch, Leo gives the PC a "Note From Leo Stahl" (accessed via Pip-Boy) reminding about the deal:

Leo's Stash
The contents of the stash always stay the same but the number is random. The stash can have any number of:
 * Bottle Caps
 * Buffout
 * Med-X
 * Mentats
 * Psycho
 * Stimpak