Medical Matters: The health and social benefits of abstaining from cannabis

Where possible, doctors strive to practice evidence-based medicine. But we are human, and not immune to the influence of the unusual or extreme case. What makes one patient stand out more than others may be a feature of the disease, the patient, or our own sensitivities.

As a medical student, the details of the first patient I saw with psychosis have remained with me. He was not much younger than me, and had started university some months earlier.

He had been admitted to hospital with a florid psychosis, convinced he was Christ.

Not unlike the many adolescents starting college this month, the freedom from home and the lights of a big city meant new experiences. Among these was the easy availability of cannabis; he had smoked a number of joints in the previous weeks.


Unfortunately, the expert opinion was that his use of cannabis precipitated the psychotic episode.

Now this was the first and only time I have seen cannabis-induced psychosis, so in the greater scheme of things it is nothing more than medical anecdote. Nonetheless, the details of the case have stayed with me to emerge from the memory banks following the publication of some related research last week.

Drug statistics

The large meta-analysis – a study of combined previous research – showed that people who are daily users of cannabis before the age of 17 are more than 60 per cent less likely to complete secondary school or to complete a degree compared with those who have never used the drug.

Published in the journal Lancet Psychiatry, the authors also found that daily users of cannabis during adolescence are seven times more likely to attempt suicide and are eight times as likely to use other illicit drugs in later life.

In this study, a team of Australian and New Zealand researchers combined data on some 3,765 participants who used cannabis from three large, long-running longitudinal studies to find out more about the link between the frequency of cannabis use before the age of 17 and seven developmental outcomes up to the age of 30.

The outcomes measured were: completing high school; obtaining a university degree; cannabis dependence; use of other illicit drugs; suicide attempt; depression; and welfare dependence. However, they did not include psychosis or a diagnosis of schizophrenia in their outcomes.

But in a linked editorial Merete Nordentoft, professor of psychiatry at the University of Copenhagen in Denmark, notes: “Cannabis use in adolescence has also been associated with increased risk of psychosis in adulthood.

“Cannabis use is associated with earlier onset of psychosis, and in patients with cannabis use and psychosis, risk of continuous psychotic symptoms is higher in those who continue to use cannabis than in those who stop.”

And a 2007 comprehensive analysis of the relationship between cannabis use and the development of psychosis in later life concluded that the risk of psychosis increased by some 40 per cent in people who have used cannabis, with the risk rising the more the drug is used.

The authors from Cardiff University and the University of Bristol estimated that some 14 per cent of cases of schizophrenia in young adults could be prevented if cannabis was not available.

There is evidence to show that brain development during adolescence can be harmed by frequent cannabis use and that cognitive functions can be reduced permanently. This impairment and the low energy and reduced initiative associated with persistent cannabis use are the likely reasons for the poor educational outcomes shown in the recent research.

With global moves to decriminalise and legalise cannabis gaining momentum, it is important to protect adolescents from gaining easier access to the drug.

Otherwise we will simply increase the numbers of young people having difficulty completing school and college, and add to those facing problems achieving social and personal maturation.

This latest research provides strong evidence that the delay or prevention of cannabis use is likely to have broad health and social benefits.