Heroin had robbed Renton of his sex drive, but now it returned with a vengeance. And as the impotence of those days faded into memory, grim desperation took hold of his sex-crazed mind. His post-junk libido, fuelled by alcohol and amphetamine, taunted him remorselessly with his own unsatisfied desire.
When you're on junk you have only one worry: scoring. When you're off it you are suddenly obliged to worry about all sorts of other shite. Got no money: can't get pissed. Got money: drinking too much. Can't get a bird: no chance of a ride. Got a bird: too much hassle. You have to worry about bills, about food, about some football team that never fucking wins, about human relationships and all the things that really don't matter when you've got a sincere and truthful junk habit.