Abbreviations and slang are more common in the drug subculture than in many others. In part this is because drug culture is so widespread throughout the world and draws from such a diversity of people, all of whom speak slightly differently. Also, since drug use and discussion isn't generally allowed in public, there's a tendency to use slang with built in obfuscation, so theoretically listeners can't be sure quite what you're talking about. Abbreviating commonly used drugs and measurements to their first letter probably arose from both of these, and has since become fairly standard and understandable argot.

Here are the ones I have personally heard used, I will add more if need be:

A - Acid
B - 2CB, known as B's or Bees
C - Cocaine*
E - Ecstasy
G - GHB*
H - Heroin
J - Joint, of Marijuana
K - Ketamine, Special K
L - Blunt, of Marijuana*
O - Ounce, of Marijuana; also sometimes for Opium*, I've been informed
Q - As in QP, a quarter pound of Marijuana
X - Ecstasy, less common than E.
Z - Ounce of Marijuana, as in "2 oz"*

Sometimes I try to make a game of seeing how many of these I can fit into conversation before somebody tells me to shut up. My best attempt so far has been "I just dropped an E, but I had some K earlier. Let's go Dutch on an O and roll ourselves up a fat J."

* Thanks to other noders for these additions.