what the heck do you think Gary Oldman was popping before he started offing people.....was never really formally explained...

Great movie, IMHO....by the way.

and had much more to do with the plot.

The type of drug wasn't important. All that mattered is you knew Gary Oldmans character was on it and central to his criminal activity.

Anyhow to answer the questions and why I went off on my riff is that it could be anything. Not likely PCP as he was never running around naked.
Cocaine? Coulda been.
LSD? doubt it as he wasn't hallucinatory
Crack? coulda been
Morphine or other pain killers? coulda been but doubt it
Heroin (which is a variation of Morphine)? don't think so. Never heard of anyone eating it.
Uppers? don't think so as he became more mellow after eating it.
Downers? coulda been

I was sure S4mypleasure had gone insane and was having a conversation with himself.