“The new review suggests that even infrequent use could raise the small but real risk of this serious mental illness by 40 per cent.… The research, paid for by the British Health Department, is being published Friday in medical journal The Lancet.”
The logical implication of this kind of reasoning is that as marijuana use increases so should the number of cases of schizophrenia. However, there are no epidemiological studies suggesting this is true. Maia Szalavitz summarizes in Salon.
“Perhaps the strongest piece of evidence to cast doubt on a causal connection between marijuana and schizophrenia is a long flat-line trend in the disease. While marijuana use rose from virtually nil in the 1940s and '50s to a peak period of use in 1979 -- when some 60 percent of high school seniors had tried it -- schizophrenia rates remained virtually constant over those decades. The same remains true today: One percent or fewer people have schizophrenia, a rate consistent among populations around the world. This is in stark contrast to studies linking tobacco smoking with lung cancer, where rises in tobacco use were accompanied by rising rates of lung cancer."
Two other things should be pointed out.
1) Rates of alcoholism (34%) and drug abuse of all kinds are much higher in the schizophrenic population (together 47%) than in the general population and no one is claiming, for example, that alcohol abuse leads to schizophrenia, indeed quite the opposite.
2) Furthermore, much of the evidence linking marijuana to schizophrenia suggests not that it causes schizophrenia per say but rather that it causes the earlier onset of symptoms in people who would sooner or later develop schizophrenia.
This is opinion of Dr Iddon.
"Dr Iddon, the chairman of the all-party parliamentary group on drugs misuse, said the study did not convince him it was time to return cannabis to class B. "I don't think the causal link has been proved. I think cannabis might - possibly for genetic reasons - trigger psychosis at an earlier age." The MP, who is also a member of the science and technology select committee, said there was a danger of criminalising "hundreds of thousands of young people" if the status of the drug was changed. "If Gordon Brown changes the class of the drug, it won't be evidence-based but for political reasons," he said.