Degenhardt, Hall and colleagues� Lancet letter is the only one this week to be classified �premium content� and thus is not available to casual �browsers� like myself. Yet once read, it seems to be almost the last word on the subject of cannabis harms. This debate has been raging in England since 1995 when Lancet famously wrote: �The smoking of cannabis, even long term, is not harmful to health.� This was both incorrect and unhelpful in a difficult area needing clarity, not bland over-simplifications. In the past, Hall has written similar sentiments � but in a more guarded and scientific manner along the lines of �For a majority of users, cannabis causes few if any adverse health consequences.� But this is a long way from saying it is harmless.
And from the current state of knowledge, summarised in this week�s insightful letters, we can now say confidently that the legal status of cannabis is not related to the rate of its use, harmful or otherwise. To quote: �Cannabis use changed at similar rates across States irrespective of these [differing] penalties. This finding strongly suggests that other factors - such as social attitudes and perceived harms - are more important drivers of consumption than penalties for use.� Regarding Australian jurisdictions, �about half have criminal penalties for possession or use, and the remainder have fines�.
As Degenhardt and colleagues rightly state, more serious than the small risk of psychosis is the 16% risk of dependence in young people using cannabis. Further, as Macleod points out in his letter, respiratory symptoms both from tobacco mixed with cannabis and even cannabis alone, also loom large as good reasons to advise young people to avoid cannabis.
Comments by Andrew Byrne ..
Degenhardt L, Hall WD, Roxburgh A, Mattick RP. UK classification of cannabis: is a change needed and why? Lancet 2007 370:1541
Zullino DF, Rathelot T, Khazaal Y. Cannabis and psychosis. Lancet 2007; 370:1540
Macleod J, Oakes R, Copello A, et al. The psychosocial sequelae of use of cannabis and other illicit drugs by young people: systematic review of longitudinal, general population studies. Lancet 2004; 363: 1579-1588
Moore THN, Zammit S, Lingford-Hughes A, et al. Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review. Lancet 2007; 370: 319-328 http://www.thelancet.com/journals/lancet/article/PIIS0140673607611623/abstract
Editorial. Rehashing the evidence on psychosis and cannabis. Lancet 2007; 370:292 http://www.thelancet.com/journals/lancet/article/PIIS0140673607611337/fulltext