i'd read the novel 'filth' a long time ago, being an avid reader of irvine welsh and was curious to see the film adaptation. 
i have to say while the film was good it deviated slightly from the book in a major way. the novel introduces a secondary character who grows as the plot progresses in the form of a tapeworm who seems to come into existence in a stream of conscious way, these thoughts are contained within the pages as an independent bubble of text becoming more and more conscious and self aware as the story of bruce, it's host develops. 
bruce himself is a corrupted police man who is falling apart, coke addicted, misogynistic, misanthropic, self destructive and heartbroken over a marriage that has ended, his course is a car crash but in the hands of welsh not only do we watch we stop the car and taste the blood. 
the film incorporated the tape worm but in a 'motif' image that appeared in bruce's imagined therapy sessions. it didn't work very well.
james mcavoy is brilliant as bruce, but it is painful to watch his decline and the inevitable results.
not pleasant viewing, not sure why i wanted to see it other than curiosity.