Last month the Department of Health acknowledged that cannabis is an “important causal factor” in mental illness. It also announced a review of recent research which suggests that cannabis not only precipitates psychosis in some patients with a predisposition to the illness but can cause mental health problems for people who would otherwise be considered low — or no — risk.

On top of this, the mental health charity Rethink called last week for an inquiry into the links between cannabis use and mental illness. And this week, a new study in the journal Neurology shows that blood flow to the brain remains altered a month after a joint is smoked.

What are parents and teenagers to make of all this news, which must add to the confusion surrounding cannabis since its declassification from a Class B to Class C drug a year ago? For most people, smoking moderate amounts of cannabis causes no problems, says Professor Robin Murray, a leading researcher in the field. But parents are understandably worried. Cannabis is cheap and easy to get hold of; much of it is stronger than it used to be. Some strains contain up to 20 per cent THC (the hallucinogenic ingredient that gets people stoned) compared with less than 5 per cent in the bog-standard stuff. Even the notoriously liberal Dutch Government is now reviewing the legality of cannabis containing high levels of THC.

However, many health professionals believe that it’s not so much strength but frequency and length of use that is the issue. Thirty-eight per cent of 15 to 16-year-olds in the UK, compared to an average of 21 per cent in the rest of Europe, have tried cannabis, and some start even younger. That is worrying in the light of recent research from the Institute of Psychiatry in South London, which suggests that adolescents may be particularly vulnerable to long-term damage. Murray, the consultant psychiatrist who coauthored the report, says: “There seems to be consistent evidence that people who start taking cannabis earlier get into more trouble, partly because they are more likely to become dependent. We found that teenagers who were taking cannabis at 15 were 4.5 times more likely to be psychotic by 26, whereas people who started taking it at 18 were only 1.5 times more likely.”

He adds: “It’s upsetting that the people with cannabis-related schizophrenia whom I see in my clinic at the Maudsley Hospital, in London, have often been highly successful children — sociable, sporty and intellectually competent — until they got into drug-taking.”

Research is also scrutinising the effect that cannabis has on brain development in adolescence. Murray cautions that nothing has been proved yet, but says: “There is speculation that if you smoke a lot of cannabis while the dopamine systems in the brain are still forming, as in adolescence, this might be detrimental. Psychosis is linked to excessive levels of dopamine in the parts of the brain concerned with emotion, and cannabis appears to increase these levels.”

The temporary effects of cannabis can be equally worrying, not least because there is considerable evidence that it impairs memory; and unlike alcohol it stays in the body for up to three months. Murray says: “One of the reasons why some young people who smoke cannabis start performing badly at school or university is that they are cognitively impaired by the cannabis lingering in their brain. A young person who smokes cannabis every day, or even three times a week, can be in a state of low-grade intoxication most of the time. However, if you stop, these adverse cognitive effects also stop.”

As with alcohol, he says, the important thing is to avoid getting stoned every night. It also reduces risk if young people delay smoking until their late teens.

Many health professionals believe that the debate about declassification has blocked the more urgent issue: better education about the risks. At the Royal College of GPs conference last month, Clare Gerada, the head of its substance misuse unit, urged GPs to talk to young people about cannabis. “I am worried about our 15 to 17-year-olds; I feel that we’ve missed the point. OK, they are not taking up smoking, because the Government’s campaign is working. But they ’re getting high on cannabis and alcohol, and the two combined do more short-term damage. We need a public health campaign on reducing the risks of all drugs taken by young people.”

Despite the UK’s high rates of cannabis consumption, it lags far behind some other countries — notably Australia — in its provision of services to help users. That may be changing. Rethink is developing a health information campaign with the Department of Health. And in December, HIT, a national drugs information and training centre, launched a website (www.knowcannabis.org.uk) and literature for people who want to cut down or quit. HIT’s director, Andrew Bennett, would like to see other services developed in the style of Australian cannabis quitlines and other cessation services. “At relatively low cost we could probably help a lot of people by developing existing tobacco cessation services and briefing GPs and practice nurses,” he says.

One award-winning drugs project in Shrewsbury, aimed at 13 to 18-year-olds, who range from “middle-class school kids to young people living on the streets”, offers a spliff-count workshop. If it’s a cause for concern, the young person is offered a referral to a specialist team, but only with his or her permission. Haroon Riaz, who runs the project, says: “Young people were confused when the law changed so I had to do my own campaign. Cannabis is not a drug to be faffed around with; we need to get on top of it.”

For more information, contact Rethink on 0845 4560455; www.rethink.org

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