Leo's Drug Habit

To begin this mini-quest the Vault 101 Dweller can ask Doc Church about the people of Megaton, then using the speech or medicine dialogue option he'll tell you about Leo Stahl's addiction to jet; alternatively you can agree to blow up Megaton for Mr. Burke (in The Power of the Atom though you don't actually have to) and then express to him concern that you can't do it; Mr. Burke will then tell you about Leo's chem habit.

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 dialogue option convincing him to quit his addiction, he'll then ask you to clean up his stash in the water treatment plant. You'll get 100xp, Karma, and Leo's stash key to a stash of meds and caps in the desk at the Water Processing Plant. Alternatively, if you ask him "What's it worth to you to keep me quiet"?, he will start dealing chems to you and you'll receive bad Karma. Later, you can kill him and loot the key to his Stash from his corpse, receiving more bad Karma for killing him.
 * Note: You only have one chance to convince him to quit through speech. If you blow it, this minor quest cannot be completed. Consider saving before attempting to use speech, or wait until your chance is 100%

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