Don't be lulled by this film's title, or the presence of that fine comedienne Jennifer Aniston; Cake is no jolly confection.

More sour than sweet, it's a drama about physical and emotional suffering whose central character would drive any carer to distraction.

Aniston plays Claire, a former lawyer whose life has been indelibly altered by a car accident. Her face and body are covered in scars, she has metal pins in her legs and is debilitated by chronic pain; the aftermath of the accident has also caused her separation from her husband, with other, terrible consequences drip-fed through the story.

Yet if Claire's problems weren't bad enough, she's exacerbating their effect with anger and self-indulgence. At the film's outset she's thrown out of a support group for mocking the suicide of another member. She's cynical, acerbic, shuns physiotherapy and is addicted to pain killers, even hiding them around her LA home so that her caring maid Silvana (Adriana Barraza) can't find them.

Thankfully for the former lawyer, she's not short of funds and has a lovely home in which to escape from the world. If she has sexual needs, she stays in bed and just summons the gardener. Silvana attends to everything else. When Claire does venture out, she's flat on her back in cars and taxis, to the point where it's difficult to distinguish between her pain and her sense of defeat.

The tone of the film is typified by Aniston's performance: stripped bare and, for the most part, unsentimental. It's not simply that the usually gorgeous actress is devoid of makeup and has taken the shine out of that famous hair; the spark has gone from her eyes. Aniston's control of her character's energy levels, keeping Claire and the film itself operating in a sort of muted state, is remarkably skilful. And she uses all of her comedy experience to deliver Claire's bitter bons mots with the minimum back swing, befitting a character at once smart and exhausted.

These glimpses of humour and occasional flashes of kindness suggest the person before the accident, and some hope, and it's Claire's journey out of the tunnel that the film addresses. Lifelines come in the form of the put-upon Silvana (a lovely performance by Barraza, full of determined kindness) and Roy (Sam Worthington), the husband of the woman whose suicide Claire had so cruelly scorned, left alone with his son and himself struggling.

Chronic pain isn't an obvious subject for a movie; the very nature of its effects, weighing people down, sucking the life out of them, isn't particularly cinematic. It has none of the potential fireworks of psychological anguish, for example. And Cake's slow pace and tone, determined by the lead character, are challenging for an audience.

But I'd rather the filmmakers held their nerve, trusting their actress's performance to hold the attention, rather than succumb to dreams and visions, and sudden, inopportune appearances of new characters to spice things up. With the exception of a road trip to Mexico for more meds, which is at least amusing, these flashes of melodrama detract rather than add to the power of the film.

Ultimately, Cake is worthwhile because of its central characterisation. In the whys and wherefores of the Oscar race, Aniston is one of the unluckiest, for being in a category full of good performances. She's played outside her customary range before, notably as a store clerk with novel romantic problems in the droll comedy The Good Girl, in 2002. Cake suggests that there is a great deal more she can offer us.