Many chess players were boozers, for instance Alekhine is said to have been so drunk during a tournament that he took a piss right on the floor.
Are any famous chess players known for playing on drugs?

GM Tkachiev (I believe he is the guy behind the provocative whychess website) was still so drunk from the previous night he fell asleep behind the board once, that's the most recent story I can think of, and that was in September 2009:
http://www.chessvibes.com/reports/drunken-tkachiev-falls-asleep-behind-the-board-makes-the-headlines

Tal I think had health problems (that eventually killed him also) in part because of heavy drinking and chain-smoking. I know of no reports that he ever PLAYED drunk in tournaments though. He seems to have been an odd and funny character in general.

Many chess players were boozers, for instance Alekhine is said to have been so drunk during a tournament that he took a piss right on the floor.
This seems to be something someone just made up, like the Paul Morphy and womens shoes thing.
no, that is inaccurate. alekhine was a heavy drinker. whether he pissed on the floor i don't know but reliable sources say so. he would also bring his cat to tournaments :)
at present there are many drugs that can improve chess skill. dexmethamphetamine and modafinil being some of the notable ones.
Are there any world class chess players who had a reputation for playing on any kind of drug, and if so, which drug?
For example, many musicians are known to use drugs, and Jimi Hendrix is believed to have played Woodstock while on LSD.
Are there any famous chess players that went into decline and the reason was thought to be drug abuse?