THUMBSUCKER — *** — Lou Pucci, Vincent D'Onofrio, Tilda Swinton; rated R (profanity, drugs, vulgarity, sex, violence, brief gore, racial epithets); see Page W2 for theaters.

"Thumbsucker's" sharp observations about overmedication, misdiagnosis of attention-deficit disorder and questionable forms of therapy make it sound like a Tom Cruise talk-show diatribe. But this dark comedy-drama is about more than just modern psychology.

In fact, in its own sneaky way, the film cleverly manages to examine the meaning of loneliness while looking at dysfunctional-family dynamics, but without becoming too overwhelming or too heavy-handed.

"Thumbsucker" is loosely based on Walter Kirn's best-selling novel of the same name (and excludes the novel's storyline about a family's conversion to the LDS Church).

The title character is Justin Cobb (Lou Pucci), an underachieving 17-year-old high school student who hasn't found a way to give up sucking his thumb, which he does whenever he feels pressured at school or at home. His overly competitive father, Mike (Vincent D'Onofrio), is exasperated as much by that as he is by his wife, Audrey (Tilda Swinton), who almost seems to encourage Justin's bad habits and lack of ambition.

So Justin turns to a variety of would-be cures, including hypnotherapy by his New Age dentist (Keanu Reeves) and medications intended for sufferers of attention-deficit disorder. While under the influence of the latter, Justin briefly becomes the star of his school's debate team and develops into an insufferable know-it-all.

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Former music video director Mike Mills packs a lot of material into the film, almost overloading it with subplots and supporting characters. Reeves does have his amusing moments, though, as do Vince Vaughn and Benjamin Bratt, who play, respectively, Justin's debate coach and a drug-addled television star.

Pucci, who resembles a member of the Culkin acting family (or at least a cousin), has a likable presence and is able to shoulder much of the dramatic load here. And both D'Onofrio and Swinton excel (as usual) as Justin's parents; D'Onofrio's later scenes with Pucci are particularly effective.

"Thumbsucker" is rated R for occasional use of strong sexual profanity, strong drug content (use and abuse of prescription medications, as well as marijuana and cocaine), some crude humor about and references to sexual and other bodily functions, simulated sex and other sexual contact, a brief scene of violence (a bicycle accident, done for laughs), some brief gore, and use of a few racial epithets. Running time: 96 minutes.