this tells me you dont have much experience with pills or cocaine.

neither make you into the sick man that the joker was in this.
i agree with knaves that it makes him more human if anything. and i see no reason (other than a fan's personal preference) that the joker would refrain from recreational drug use. he steals, lies, murders, destroys, cripples and generally causes chaos and anarchy at every turn but he draws the line at ingesting illegal substances? not buying it. in fact i saw nothin' in this book that was out of character for the joker imo.
couple of things i noticed. a bat like figure is seen a few times up amongst the roof tops of gotham. the first (and most obvious) is when j is first released. on the page following the splash page (of him at the gates of arkham) second panel. crouching just above the gargoyle is a horned figure. in the next panel joker looks back over his shoulder. next panel lightning strikes and the figure is gone. next panel joker "salutes the city". is he really saluting bats who he knows is watching him and who joker thought he glimpsed?
when joker tells jonny to hand him the shovel in the trunk of the firebird, jonny replies "how'd you--" implying that the joker knew it was there. isnt it entirely possible that joker had been in that car before? maybe he even put that shovel there at some point in the past? i mean the joker was monty's boss and the grin and bear it was his club. could it be that it was also his car? or maybe its just common practice amongst gangsters to keep spare spades in the trunk for rainy days? after all you never know when you might have to dig a hole at a moments notice.