Although it looks thoroughly off-putting, the feces in the Worst Toilet in Scotland scene was actually made from chocolate and smelled quite pleasant.
Ewan McGregor read books about crack and heroin to prepare for the role. He also went to Glasgow and met people from the Calton Athletic Recovery Group, an organisation of recovering heroin addicts. He was taught how to cook up heroin with a spoon using glucose powder. McGregor considered injecting heroin to better understand the character, but eventually decided against it.
Kelly Macdonald, in her naiveté, invited her mother and brother to the set while filming her sex scene with Ewan McGregor.
For the close-up shots of Ewan McGregor injecting himself with heroin, a prosthetic arm was constructed by the make-up department, complete with pulsing veins, smack tracks and small pockets of blood that would appear when the skin was punctured by a hypodermic needle.
Due to the skimpy budget, most scenes had to be shot in just one take.