Short bio: Computer Scientist, FOSS supporter (read more)
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It is a frightening conclusion, and one that is already starting to shape the debate over the legal status of cannabis. In the UK, van Os's findings have been seized upon by politicians, tabloid newspapers and mental-health lobby groups who want drug laws tightened up. But their case is far from made. Some researchers say there are potentially fatal flaws in the research, and that it would be a serious mistake to change the law based on an as-yet unproven theory. So who is right? The answer, it turns out, is more complex than it first appears.
Compared with substances like heroin and crack cocaine, cannabis is seen by many people as relatively harmless. Several European countries take a lenient view of its use, and in the UK marijuana was downgraded from a class B to a class C drug last year, meaning people caught with small quantities are not usually arrested.
But doctors have long known that taking a lot of cannabis over a short period can occasionally cause temporary symptoms of psychosis, one of the hallmarks of schizophrenia (see "Psychosis explained"). The question of whether the drug has long-term mental health consequences, however, is much more contentious.
What should be done about it, however, remains an open question. Van Os advocates that teenagers with a personal or family history of mental illness be urged to steer clear of the drug. He also advocates legal changes: governments should focus on keeping cannabis out of the hands of teenagers and outlawing extra-strong varieties of cannabis, such as skunk and white widow.