So Renton has decided to quit the gear. He goes to see a dealer (played by Irvine Welsh, the author of the book) to get hold of some methadone, but all he has left are some heroin suppositories. On the way back home where he plans to start his detox, he gets caught short and has no choice but to wander into the nearest loo. Surreal scene depicting the utter desperation of drug addiction/withdrawal.