Leo's Drug Habit

Leo's Drug Habit refers to an unmarked quest in Fallout 3.

Obtaining the quest
By either talking to Doc Church, Lucas Simms, or Mister 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 express your concerns to Mister Burke that you can't do it, and he will tell you about Leo Stahl's chem habit.

Completing the quest
Go talk to Leo Stahl in the The Brass Lantern during the day or in the Water Processing Plant at night and ask him about his Chem habit.

Good Karmic Path
The good Karma way is to use the Speech dialog option to quit his addiction, he'll then ask you to clean up his stash in the Water Processing Plant. You'll get 100xp (unmodified, plus 10 from the speech check), good Karma and his key to a stash of Chems and Caps in the desk at the Water Processing Plant. However, even as he is claiming to be cleaning up, it appears that he still uses drugs as he can be found in the Water Processing Plant. When asked with a dialogue option, "What are you doing here?", he will respond nervously and claim that he is doing nothing. He will still come to the Plant even if the player has raided his stash, so it is unknown why he comes here.

Andy Stahl's attitude towards the player will become more friendly if this path is chosen. Andy will remark that his brother means the world to him, and that the Lone Wanderer may be the only reason he's still alive.

Neutral Karmic path
Initially choose the 'good' path choice, but do not choose the Speech challenge option. Instead, choose the 'I never thought about it that way' option. Leo adds the "Note From Leo Stahl" to your Pip-Boy and will now be available as a chem dealer during his evenings in the water treatment plant. This choice provides no Karma gain or Loss, does not give any XP reward, but does enable a conveniently located chem source in Megaton.

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 you, and then you will receive bad Karma. Later, you can kill him and loot the key to his stash from his corpse; you'll obtain more bad Karma for killing him.

Upon taking the Bad Karmic Path, Leo gives you 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
 * Stimpaks
 * Jet