The Advisory Council on the Misuse of Drugs (ACMD), on which Professor David Nutt sits, has an unfortunate history in relation to cannabis. In 2002, it boobed by advising David Blunkett, then home secretary, that there were no serious mental health consequences of cannabis use; the council had done a sloppy job of reviewing the evidence. Since that time, they have been trying to regain credibility, and now accept that heavy use of cannabis is a risk factor for psychotic illnesses including schizophrenia. However, Professor Nutt's comments demonstrate how difficult it has been for some members of the committee to accept their error.

Professor Nutt states that, in 2007, the ACMD were asked to review the situation again because "supposedly, skunk use had been increasing and it was getting stronger". In fact, the ACMD itself concluded that street cannabis was getting more potent and a Department of Health survey has shown that skunk has been taking an ever-larger share of the market.

Professor Nutt states that "there has been a lot of commentary and some research as to whether cannabis is associated with schizophrenia." It is crystal clear that people with schizophrenia use more cannabis than the general population; there is no dispute about this. The question is whether the use of cannabis contributes to the onset of psychosis including schizophrenia in a causal manner. Here the evidence, although not yet conclusive, has been mounting steadily over the past six years.

Professor Nutt contrasts a 2.6 fold increase in risk of psychosis carried by using cannabis with a twentyfold increase in risk of lung cancer if one smokes cigarettes. Unfortunately, he is not comparing like with like. The twentyfold increased risk is not carried by just being a cigarette smoker but rather by being a long-term heavy smoker. For cannabis, the risk of psychosis goes up to about six times if one is a long-term heavy cannabis smoker.

Next Professor Nutt claims that the incidence of schizophrenia is falling while consumption of skunk has been rising. Sadly, the paper he points to is a study of diagnosis in general practice and we know that GP records on psychosis are far from accurate. The only good longitudinal data on the incidence of schizophrenia in the UK comes from south London, where the incidence doubled between 1964 and 1999. There are probably several factors contributing to this but abuse of drugs is likely to be one.

Personally, I care little whether cannabis is classified as a class B or class C drug. Fourteen year olds starting daily cannabis use do not agonise over its exact classification; many do not even think it is a drug and few have any knowledge of its hazards. By comparison, most adults in the UK drink alcohol in moderation, but do so in the knowledge that drinking a bottle of vodka a day is likely to be injurious to health, and few are in favour of daily drinking from age 14 years.

Both Professor Nutt and I agree that what we need is a major educational campaign to inform the public about the risks associated with heavy use of cannabis particularly in early adolescence. Fortunately, there has been some progress in public understanding and, as a consequence, use of cannabis has been falling for the past five years.