July 26, 2007

experts comment on a Lancet study linking cannabis use to an increased risk of psychosis

The paper reported that cannabis users are 40% more likely than non-users to suffer a psychotic illness such as schizophrenia.

Professor David Nutt, Head of psychopharmacology, University of Bristol, said:

“Cannabis is unquestionably harmful and may cause psychotic symptoms or even a more enduring illness in a few percentage of users. However as our recent paper in the Lancet** shows in public health terms it is very much less addictive or damaging than either alcohol or tobacco. The idea that reclassification upwards will do anything to reduce psychosis is naive and runs the risk of perversely inflicting even greater suffering – through increasing criminal sanctions – on vulnerable individuals already afflicted with mental illness.”

Dr. Zerrin Atakan, Consultant Psychiatrist/Hon. Senior Lecturer, Institute Of Psychiatry, Kings College London , said:

“It is good to see that what we have observed and suspected clinically for some time is gathering increasing evidence based recognition. Even though we still need to find more biological evidence, there is now enough scientific knowledge to inform young people about the risks involved in smoking cannabis. I really expect the government to now start a more effective educational campaign on the subject, starting at primary school level.”

Professor Leslie Iverson, University of Oxford, said:

“Despite a thorough review of the subject the authors admit that there is no conclusive evidence that cannabis use causes psychotic illness. Their prediction that 14% of psychotic outcomes in young adults in the UK may be due to cannabis use is not supported by the fact that the incidence of schizophrenia has not shown any significant change in the past thirty years.”

Professor Robin Murray, Institute of Psychiatry, Kings College London, said:

“This is a very competent and conservative assessment of what research studies tell us about the relationship between cannabis and psychiatric disorders.

“I agree with the authors that the evidence concerning depression and anxiety is uncertain, largely because there have been insufficient studies.

“I also agree that the evidence does indicate that cannabis use increases the later risk of psychotic symptoms and schizophrenia, and that the heavier the use, the higher the risk.

“This meta-analysis is, of course, concerned with historical data, and the studies considered examine the effects of the cannabis that was available from the 1960’s through to the 1990’s. Therefore this report cannot tell us whether the risk is higher with the use of the skunk-like preparations which are now widely available, and which contain a higher percentage of the active ingredient tetrahydrocannabinol (THC). My own experience suggest to me that the risk with skunk is higher. Therefore their estimate that 14% of cases of schizophrenia in the UK are due to cannabis is now probably an understatement.

“Finally I agree with the authors that epidemiological studies such as this cannot take our understanding much further forward. What we need are experimental studies examining the effects of cannabis in humans and in animals. Unfortunately neither the Department of Health or the Medical Research Council supports any significant programs of research into these areas.”