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Early Cannabis Dangers Quantified

Girl being offered marijuana

Early Cannabis Dangers Quantified: iStockphoto

By Stephen Luntz

A study at the Queensland Brain Institute has confirmed the dangers of early cannabis use, revealing that users who start by the time they are 15 years old are three times as likely as non-users to experience psychosis by the age of 21.

A study at the Queensland Brain Institute has confirmed the dangers of early cannabis use, revealing that users who start by the time they are 15 years old are three times as likely as non-users to experience psychosis by the age of 21.

While the association of schizophrenia and cannabis use is well-known, it has not been easy to prove that the drug consumption is a cause of mental illness rather than a form of self-medication.

However, the University of Queensland’s Prof John McGrath has now drawn on a longitudinal study of children born at Brisbane’s Mater Hospital in the early 1980s. Besides providing information on the timing of cannabis use, the Mater Hospital study included 228 sets of siblings.

“Looking at siblings is a type of natural experiment – we found the same links within the siblings as we did in the entire sample. The younger you are when you started to use cannabis – the greater the risk of having psychotic symptoms at age 21,” McGrath says.

Clinical schizophrenia is rare, affecting just 1% of the population. However, the study also took into account larger numbers of people who heard voices or reported delusional experiences. Across all three categories, those who had started smoking cannabis 6 years earlier were between two and four times as likely to be affected as those who had never smoked. More recent cannabis use resulted in smaller increases in psychosis.

The Mater Hospital study did not include a measure of cumulative consumption, raising the possibility that early users also use more heavily, and that this might be the true cause of the association. However, McGrath says other studies have considered this. “We don’t interpret our results in isolation.” While these other studies lack some features of the Mater sample, McGrath says that “they confirm early adolescence is a particularly vulnerable period for the brain”.

“This is the most convincing evidence yet that the earlier you use cannabis, the more likely you are to have symptoms of a psychotic illness,” McGrath says. “The message for teenagers is: if they choose to use cannabis they have to understand there’s a risk involved. Everyone takes risks every day – think of the sports we play or the way we drive – and people need to know that we now believe that early cannabis use is a risk for later psychotic illness.”