Why do around half of young people with a severe mental health problem use street drugs? Think for a moment of a typical scenario for the early onset of psychotic symptoms. A young man is away from home at college and studying hard for his exams. He's feeling under a lot of pressure and he starts hearing voices. His first reaction is fear - could he be going mad, is he going be locked up? Does he go to his GP for help? Does he talk to his friends or his parents? Possibly, but more likely he turns to alcohol or drugs to block out his fears and have some relief from his symptoms.

Angela Jones' (not her real name) son started using drugs at the age of 15. Over the next 18 months he became extremely ill, culminating in him being sectioned. She says: "He was using drugs - mainly cannabis. We knew that something was going wrong. But we didn't know whether this was normal adolescent behaviour or not. We know now that he was using the drugs to quell what was happening in his head. There were a lot of voices. He said, 'The drugs on the street are far better than the ones they're giving me.'"

Whatever relief users find they provide, it is widely acknowledged that street drugs, including cannabis, crack and cocaine, can exacerbate psychotic symptoms.

There is also a growing body of evidence that for some young people cannabis may actually cause psychosis. Studies from Sweden and the Netherlands indicate that regular cannabis users are several times more likely to be subsequently diagnosed with schizophrenia than those who don't use the drug.

Robin Murray is a consultant psychiatrist working at the Maudsley Hospital in south London. He points out that, while people with schizophrenia do not take more alcohol, heroin or ecstasy than the rest of the population, they are more than twice as likely to smoke cannabis regularly. He believes the increasing availability of the drug is one of the reasons why the incidence of schizophrenia has doubled in south London since the 1960s.

So while for many young people a spliff on a Friday night is as normal as a can of lager, for an unfortunate minority, it may spell the beginning of severe mental illness. Exactly what makes some cannabis users develop psychosis is unclear, but it seems to be a mixture of genetic predisposition and social and environmental factors.

Rethink (previously the National Schizophrenia Fellowship) is trying to tackle the ignorance surrounding severe mental illness by getting service users and staff to talk to young people at college and university. Mike Hartley, operations manager for the charity in Yorkshire and the north east, says: "The idea is to inform young people so they don't hide symptoms that are causing them concern.

"Service users talk about their experience and encourage people to go to their GP or ring a helpline. It's about encouraging people to be open and honest."

Early intervention can dramatically improve the life chances of a young person who is developing psychotic symptoms, as well as reducing the risk of them of developing a serious drug dependency. Andrew Higgins, Rethink operations manager for the south west, says: "There's a strong relationship between a short duration of untreated psychosis and a higher level of recovery. If symptoms are treated early, someone has a good chance of getting back to where they were before they developed symptoms, in terms of their social functioning, concentration, and ability to hold down a place at college or a job. Hopefully, we reach people a long time before they get to an acute ward." The reality, however, is that even once they are referred for treatment, teenagers may have to wait for up to 18 months.

Teenagers often fall between child and adolescent mental health services (CAMHS) and adult services. Higgins says: "A colleague with a daughter who was using drugs and developing psychosis took her to children's services and was told, 'We don't do psychosis', and that he needed to take her to adult services. When he contacted them they told him to go back to CAMHS."

As well as the relatively small number of young people who develop psychosis while still in their teens, there is a much larger number of children with a range of mental health problems who use drugs as a way of trying to cope with difficulties in their lives.

Dr Mirza, consultant psychiatrist with the South London and Maudsley NHS Trust and senior lecturer at the Institute of Psychiatry, says that in the country as a whole, services for teenagers that combine treatment of both drug dependency and mental illness are almost non-existent.

He agrees that specialist inpatient care for teenagers is needed in the most extreme cases. At the moment most severely mentally ill teenagers who are using drugs have to be admitted to an adult ward or private clinic, if they are treated at all. But he believes the first priority should be to develop multi-disciplinary, community-based support.

He explains: "There's a strong association between trauma and substance abuse - numbing your brain with anything you can get. Many of the young people I work with who have disciplinary problems and are using drugs - the so-called 'bad kids' - also have a lot of sadness in their lives. Some have suffered neglect or abuse, or post-traumatic stress disorder from war or conflict. They use drugs as a way of dealing with their problems. Mental health professionals have a role here, although by and large people in CAMHS don't feel they have the ability to deal with substance abuse problems."

Mirza says many of these young people have multiple needs and chaotic lifestyles. "If you want to help them you have to go out to them. We are trying to develop an extensive outreach service. We have created a network of professionals - the service is based in CAMHS but we have people from social services, the voluntary sector, education and youth offending. If you look at service developments in other countries what seem to be successful are services which meet their needs together," he says.

Of the 400-500 young people aged between 14 and 18 referred to the Lambeth CAMHS team in south London, around a quarter have significant substance abuse problems - at least 100 a year. Yet in many parts of the country there is no provision at all. Hartley says that in the 15 local authority areas he covers, only three or four have any service for teenagers with a dual diagnosis. He says, "There's definitely a move towards early intervention and involving families, but it's still embryonic."

Like other agencies working with homeless people, the London-based charity Centrepoint picks up some of the thousands of young people whose mental health needs have not been addressed. Maura O'Brien, co-ordinator of the multiple needs team, echoes the message that young people use drugs as a form of self-medication. "Often they find the drug or alcohol gives them some relief from their symptoms. We help them to look at reasons why they are using a drug, and see what makes them feel better and worse," she explains.

In line with new guidelines from the Department of Health on dual diagnosis promoting integrated treatment, O'Brien believes an integrated approach to supporting people with mental health problems is the most effective. "We often find people have to go around a number of different services. People with crack psychosis are often told by mental health services they have to give up the crack. They don't find that very helpful. Clinical outcomes tend to be better with one practitioner."

But while the inadequacy of provision for adults with a dual diagnosis is slowly being addressed, children's services still have a long, long way to go. Yet there's no doubt that early intervention is the answer. Jones says: "With Tom, the signs were there when he started secondary school. At 15 he was using cannabis as a form of self-medicationÉ it's so important that we reach people early."