Animal, Soho Theatre, London

This play is a peculiar beast. Its author, Kay Adshead, won justified acclaim for The Bogus Woman, a powerful drama about the nightmarish experiences of an asylum-seeker. In this new piece, she directs her attention to the dangers posed by the psycho-pharmaceutical industry and to the ethics of experiment.

Animal is set in a London treatment centre, where an old homeless tramp called Pongo (Richard Owens) is one of the "volunteers" in a scientifically controlled anger-management drug trial. Medicated and monitored by a female doctor and a male psychiatric nurse, this man, who has lived rough for years, begins to lose his antisocial aggression and to reconnect with the intelligent, talented individual he may have been before his breakdown.

Adshead's handling of this part of the story is sensitive and thought-provoking. It pulls you into the predicament of a man who is still scarred by the early death of his mother and so pathetically famished for love and sexual experience that he develops a romantic fixation on Fiona Bell's severe and insecure doctor. After a weird episode in which he butchers a swan in the park outside and then puts the viability of the trial in jeopardy by feigning drug-caused blindness, his medication strength is boosted to the point of catatonia.

Rather as in A Clockwork Orange, with its arguments about scientific control and free will, Animal raises disturbing questions about the morality of interference. To remove someone's capacity for anger is also to take away their human right to dissent, and comes dangerously close to the view that nonconformity is a clinical condition that needs "curing" with drugs.

Lucidly directed by Lisa Goldman, with ghostly CCTV footage reinforcing the atmosphere of surveillance unease, this three-hander attempts to dramatise the relationship between the tests in the treatment centre and the mass demonstrations against them in the park outside. Upping the stakes but lessening the impact of the piece, Adshead has set the proceedings in an alternative-reality dystopian England, a tyranny that's in a state of constant war, with protest gatherings brutally put down by the police. Reporting on these offstage events is Elmo (beautifully played by Mark Monero), the male nurse who moonlights as a cocky stand-up comic and whose 10-minute set here is reprehensibly enjoyable in its jaw-dropping chauvinism. The play charts the growth in Elmo's political awareness, but there's so little hard detail in its doom-laden painting of the broader ideological picture that the connections between this alternative England and ours (with the demonstrations against the war in Iraq) are obscured in a melodramatic haze.

The doctor's thinking comes across as somewhat muddled; how deliberately, it's hard to tell. In a tense but contrived confrontation with Pongo, who has made off with her tiny baby, she tries to talk her patient into reasonableness by revealing that life has been hard for her too. She reveals that she is a Czech who, as a little girl, watched the Soviet tanks roll in and witnessed the lynching by her people of a young Russian soldier. It's perfectly possible to believe that this would have had a traumatic effect on her. It's less easy to credit that it would have given her a lifelong hatred of the mob, regardless of the justice of her cause. State suppression of mass protest, in the interests of "peace", is surely not something you would ever find an East European intellectual even supporting, let alone working to find a chemical weapon for. This kind of overkill leaves what could have been a properly alarming play sounding shrilly alarmist.

To 27 Sept (020-7478 0100)