Sharon Devalda hears voices. They are inside her head, usually male, and keep up a running, tirade of racial abuse, insults and obscenities during most of her waking day, and often long into the early hours. 'Slag, black bastard . . . that sort of thing,' she says.

In common with one in 100 of the population, Sharon has schizophrenia. She cannot stop the voices, nor can the cocktail of drugs she takes every day. The drugs, including a slow-release tranquilliser that is injected once every two weeks, have little effect on the severe depression that cripples her life. They do, however, have side-effects, for which she needs yet more drugs.

Almost 100 years after schizophrenia was first identified, there is still no known cure, or even an established cause, for the most common form of severe mental illness. Treatment for many is centred on control, not cure. But the drugs that replace the asylum's bars at best offer partial relief, at worst have devastating side-effects that mimic some of the symptoms of Parkinson's disease or reduce the body's immunity.

No wonder that care in the community poses such great problems. In addition, there is a widespread perception that schizophrenics are violent. Sharon says: 'Everybody these days thinks we're going to kill them. Yet most of the time we sit around being depressed, drinking coffee and smoking cigarettes in between taking the drugs they give us. The drugs slow you down. I used to read a lot, but I don't any more. The days go by in a haze.'

Sharon has now made a remarkable video diary of her daily life that will be shown tomorrow on BBC 2. She hopes it will demonstrate that the most important support she has in the community comes not from the professionals, but from fellow sufferers.

The everyday life of Sharon, who lives in Manchester with Micky, her husband and also a schizophrenic, is regulated by anti-psychotic drugs which have changed little in design and effect since they appeared in 1952. Clozapine, introduced in 1990, was the first anti-psychotic drug for 40 years to show that these medicines could be improved.

Now, seven new products are on trial, triggered by the need for improvements and the knowledge of the rich pickings to be had. Schizophrenics have to take drugs all their life and the latest product costs the NHS pounds 3,000 a year for each patient. Symptoms can go away for a few patients, only to return later.

Professor Robin Murray of London's Institute of Psychiatry says it is the most exciting time in the field for many years. 'Once Clozapine was put on the market and it was realised that anti-psychotic drugs could actually be made better, all the companies have been racing to find better anti-psychotics,' he says. 'I think that in five years' time we will probably not use any of the present-day drugs other than Clozapine. They will all be swept away by better drugs.'

The drugs work by slowing the communication network in the brain, damping down communication between cells. 'As the message goes down the cell, it has to jump to the next cell and it does that through chemical transmitters,' says Dr Tim Crowe, a leading specialist on schizophrenia, from Northwick Park Hospital, north London.

'What the drugs do is to block the receptor on the next cell, and that has the effect of damping down the response. Its effect is to get rid of hallucinations.'

The drugs' action centres on dopamine, a brain chemical associated with physical movement and energising behaviour. The drugs act as blockers, slowing down the messages passed to the dopamine receivers in the cells.

Existing drugs, however, only tackle the so-called positive symptoms of schizophrenia - hallucinations, voices, delusions and bizarre ideas.

'The drugs we have get rid of these positive symptoms in around 80 per cent of people,' says Professor Murray, 'but for the negative symptoms - the loss of drive, loss of enthusiasm, the withering of personality - the drugs do nothing.'

Some of the new drugs on trial will also treat the negative side, and have fewer side-effects.

In the meantime, new ways of brain imaging are shedding more light on the processes involved. 'One can now show that when people hear voices, there is excessive activity in the part of the brain which is usually concerned with 'inner' speech,' says Professor Murray. 'When people rehearse a speech, for instance, in their minds, we call that inner speech. You and I know we are talking to ourselves, but the person with schizophrenia misperceives their own inner speech as being external voices.

'People with schizophrenia wonder if the voices are the result of telepathy, or radio waves, or is it a neighbour trying to harm them? They can become paranoid: they think the world is out to get them. And if you think people are about to attack you, you may respond.'

The cause could be genetic or the consequence of early brain damage. 'If you think of the brain as a computer, in the schizophrenic the wiring is 99 per cent correct but there is a fault,' Professor Murray explains. 'Like the faulty computer, most of the time it works OK, but if you stress it too much, it may crash.'

In her video diary, Sharon sometimes breaks down before the camera turns and doesn't want it to carry on. But she does, and she has finished it. And she now has a title for it. 'You can call it the Diary of a Nutter,' she laughs.

'Video Diaries', BBC 2, 9.30pm, Wednesday 14 September.

National Schizophrenia Fellowship advice line: 081-974 6814. NSF, 28 Castle Street, Kingston upon Thames, Surrey KT1 1SS.