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The White House Office of National Drug Control Policy released a “study” last week arguing, essentially, that teenage pot smoking causes mental illness. The NDCP seems to trot out one of these reports every year or so, often citing data showing an increased risk of mental illness among youth who smoked reefer. I called bullshit on that alleged causal link last week. My argument is better made in a new study from the British Home Office, cautioning against the temptation to draw find a cause and effect relationship between cannabis use and increased risk of mental illness.

“An association between cannabis use and the subsequent development of a psychotic illness does not necessarily indicate a causal relationship in either individuals or populations. The onset of schizophrenia usually occurs in the late teens or early twenties; and it is at this age that cannabis use is most prevalent. A temporal association – which may not necessarily be a causal one – is therefore almost inevitable”

The authors note later that other factors might be at play, “such as a common predisposition to schizophrenia and also to cannabis use.” Meaning, people destined to develop schizophrenia may also be the kind of people who smoke pot in high school. Not so hard to believe. As for the potential impact of the mental health menace that is marijuana, the study notes that very few young pot smokers (in the UK anyway) will go on to develop a psychotic illness. According to their research, “around 5,000 young men, or 20,000 young women, would need to be prevented from using cannabis to avoid one person developing schizophrenia.”