I'm trying to think of all the conditions under which you shouldn't watch Channel 4's adaptation of the acclaimed written-by-a- former-mental-patient novel, Poppy Shakespeare. It's quite a long list.

You wouldn't want to watch it if you are feeling depressed, for instance. With a drama that oscillates between uneasy, nausea-green NHS corridors and what will almost certainly be the most squalid flat you'll see on TV all year - it makes the homes in Shameless look like Nigella's pink kitchen - you can't help but feel a little sombre, after a while.

Similarly, you wouldn't want to watch it if you, or a loved one, were at the very start of seeking psychiatric treatment from the NHS. Let's face it - dramas about mental health problems are never merry at the best of times, and this one invokes all the big touchstones (Kafka, One Flew Over the Cuckoo's Nest) you sincerely hope you will never be citing when describing what happened after your big nervous breakdown.

It's your classic nightmare, really. A perfectly normal woman - Poppy Shakespeare - is sectioned into the Dorothy Fish mental health unit, insisting that she is “f***ing sane, all right?”

Meanwhile, patients in Dorothy Fish who are still troubled get forcibly discharged, and habitually kill themselves within 24 hours. Psychiatric counselling consists of little more than 15 patients sitting in a circle, smoking roll-ups and screaming abuse at each other.

And the social workers are presented as a chilling blend of simpering patronisation and evil machination - an effect made even more visceral by casting Raquel, Del's shrewish wife in Only Fools and Horses, as one of them. If your notion of reality were becoming fundamentally unbalanced, the first port of call for many paranoid delusions would, I think, be that a major cast member of Only Fools and Horses was in on the plot. I tell you, if I'm ever going a bit woo-woo and a junior registrar who looks like Uncle Albert walks into the room, I will be leaving, head first, via the window.

Looking on the bright side, though - as we must, if we are not to all end up sitting on top of a battered filing cabinet on a mental health ward, drinking handfuls of black-market anti-psychotic mixed into a five-litre bottle of orange juice and vodka - Poppy Shakespeare is a very good drama. You might feel awful at the end of it - pretty terrible, to be honest - but you will be glad you watched. Most of this is down to the Bafta award-winning Anna Maxwell Martin (Becoming Jane, Bleak House), who, unless Dame Judi Dench suddenly turns in a career-best as Mick Ronson in a surrealist David Bowie biopic, will probably get her second Bafta for Poppy Shakespeare.

With her blunt, childlike face - defenceless apart from the eyes - she plays “N”, a life-long psychiatric patient, welded by sweat and sebum into a red puffa-coat, and Poppy Shakespeare's guide to “surviving” in a mental health unit.

As in the novel, it is “N” who stops Poppy Shakespeare being all “If you wish to discuss any of the issues raised in this programme, there is a phoneline”-ish - and, instead, makes it about one unreliable, wonky narrator taking pride in her expertise at “dribbling” (being mentally ill). Her signature speech - “Weren't nobody else in the world ... not no one at all, alive or dead or both or neither, known as much about dribbling as I did” - sets the drama's attitude and pace. This is One Flew Over the Cuckoo's Nest relived in an inmate's head - a world where “Assessment Day” is rendered as a musical number, patients swap their meds for cigarettes, and the hospital stretched “up into the clouds ... and if you ever went above the eighth floor, you never came back”.

Maxwell Martin is pretty extraordinary in the role. She shifts through the course of the story from a filthy, asexual, introverted, medicated urchin into a bitch in lipstick and a leopardskin coat, and then back again - but all the time retaining her core person of an essentially unempathic child, raised by a series of grey buildings. Opposite her, as Poppy Shakespeare, Naomie Harris (the voodoo priestess from Pirates of the Caribbean) feels a few scenes short of convincingly losing her mind, but certainly doesn't get in the way of the story's self-destructive brio, or singular, unsettling tone.

Just don't watch it alone, on a rainy night, after a bad football result.

Poppy Shakespeare, Mon, C4, 9pm

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