Unflinching in its portrayal of young heroin users, "Trainspotting" is almost impossible to turn away from - albeit in the same way that car headlights are irresistible to wildlife.

Of course, that doesn't necessarily make the film an instant classic. Its four main "anti-heroes" are such unsympathetic losers that it is difficult to care whether they live or die.

Also, the film's constant profanity, graphic drug use (most being injected intravenously), explicit sex scenes and some nauseatingly vulgar gags sometimes make "Pulp Fiction" look like an exercise in restraint.

That said, the film does have a sort of rude, prickly charm when it tries to be straightforwardly funny (as compared to the more darkly humorous bits that are just plain painful to watch). But there are too few of those parts for the whole thing to work. Instead, it's an inconsistent mix of comedy and tragedy.

The movie, which loosely adapts Irvine Welsh's bitterly comedic novel, follows Mark Renton (Ewan McGregor), a young Scottish heroin addict. Along with his friends Sick Boy (Jonny Lee Miller) and Spud (Ewen Bremner), Renton steals from other drug users and the elderly to support his drug habit.

But after he and Spud are caught shoplifting, Renton is forced to clean up his act (shown vividly in a terrifying scene where he suffers nightmarish hallucinations) and even gets a job as a real estate agent in England. That all comes to an end when he's visited by the psychotic Begbie (Robert Carlyle), an alcoholic who's on the run from the police and who eventually gets him involved in a drug caper.

In between, Renton manages to get involved, sexually, with a 14-year-old schoolgirl, while Spud - amped alternately on drugs and alcohol - destroys his relationship with his girlfriend and sabotages a promising job interview with a prospective employer.

Director Danny Boyle - who was also responsible for the similarly uneven "Shallow Grave" - wisely tries to balance his portrayal of heroin addiction. Besides some weirdly euphoric sequences depicting the junkies' altered states of consciousness (including a befuddling one where Renton climbs inside a toilet to look for drugs), Boyle also shows the devastating physical toll the drug takes on their bodies and minds.

The performers are up to the difficult task of playing addicts, especially Bremner, who portrays a particular naive and guileless junkie. Miller, an English actor - who played an American in "Hackers" - uses his thick Scottish accent (including a devastating Sean Connery impression) quite convincingly.

Unfortunately, John Hodge's screenplay makes them all utterly unlikable. McGregor, who made his debut as the smarmy journalist in "Shallow Grave," fares the best of the lot, but any sympathy we might muster for his character ends abruptly when Renton shoots up immediately after he finds out a friend's baby has died.

As stated, there are some very funny moments, in particular, Spud's job interview scene (where he blurts out the first thing that comes to mind) and another bit where the sharpshooting Renton gets a bulldog to attack his owner, a skinhead.

However, there are some cruelly unfunny and tasteless parts. For example, the scene where Spud soils some bedsheets, which is discovered by his girlfriend's parents, is just sickening.

"Trainspotting" is rated R but really puts that rating to the test with its ample non-stop profanity, drug use, graphic sex, nudity, violence and vulgarity.