He drank an ultra-cheap whiskey called "White Viper" that some liquor store owner found in his basement and decided to sell to the local homeless people. They never really explain why the stuff causes this.
I'm honored to say that the "stunt double" who actually gets sucked into the toilet has been one of my best friends for nearly 20 years. He still has a few prop bottles of "Viper", by the way.
The first Synapse DVD release of Street Trash actually comes with label stickers so you can "make your own" bottle of Viper. This'd be the single disc release, not the 2-disc that came out about six months to a year later.