He's pretending to do drugs.... If you listen to what he's saying while he is doing it you'll understand. Re-watch the scene, you'll get it. My girlfriend asked the same thing while we were watcing it.
Here's what he says:
"Ignore the nightmare in the bathroom. Just another ugly refugee from the love generation. My attorney had never accepted the notion, often espoused by former drug abusers, that you can get a lot higher without drugs than with them. And neither have I for that matter."
Then he starts tripping.....