Cannabis use and psychosis are linked, but study finds age matters

Cannabis linked to earlier onset of psychosis, with gap wider in older adults In short: A new meta-analysis from Australian researchers has looked at 149 international studies and suggests the age of psychosis onset for cannabis-users is 2.5 years earlier than non-users. The study's authors note there's a growing body of evidence that harm may be cumulative, with ongoing cannabis exposure potentially lowering the "threshold" for psychosis onset. What's next? Now, experts say there is a need for greater health warnings about the potential harms of cannabis, with more restrictions on high-potency products available in Australia. Cannabis-users who develop psychosis in their 30s experience symptoms more than six years earlier on average than people with the condition who don't use the drug, according to new research. These figures are in sharp contrast to younger age groups, with cannabis-users in their early 20s who develop psychosis doing so on average about 1.6 years earlier than non-users, and no evidence of earlier onset in those under 20. The study's senior author, University of New South Wales (UNSW) researcher Matthew Large, said the association between using cannabis and the early onset of psychosis had already been suggested by previous studies. But in this new large-scale analysis, published today in Biological Psychiatry, Dr Large said researchers were able to examine an "enormous amount of data" and use "fine grain analysis" to look at age sub-groups for the first time. People experiencing psychosis often feel disconnected from reality, and struggle to distinguish between what's real and what isn't. The older the age group was, Dr Large explained, the larger gap there was between cannabis-users and non-users in regards to the age when symptoms of psychosis first appeared. "It was widely assumed that the earlier age of onset of [psychosis] mostly affected young people. This study shows, contrary to that expectation, that it mostly affects people in their late 20s and over 30s," he said. "That is a finding that has not been demonstrated in any other study, [it] hasn't even been suggested." The study's lead author, UNSW academic and medical student Carly Stevens, said this suggested the harm could be "cumulative". "There seems to be this predominant idea of the adolescent brain being very vulnerable, which is true," she said. Using cannabis at a young age can disrupt neurodevelopmental processes in the adolescent brain, influencing how the brain develops, affecting the memory, attention and growth. "But also this prevailing myth that if you wait until your brain develops, and then you engage in these kind of behaviours, that you'll be OK," she said. "But our findings likely show that in terms of the onset of psychosis, it's not younger people that are at risk, but actually older people." Earlier in life, Ms Stevens said genetic vulnerability and neurodevelopmental factors could play a greater role as there was very little time for cannabis to have an influence. But, she said, ongoing cannabis use could lower the threshold for when symptoms of psychosis first appeared later in life. More data, same results Dr Large, who is a clinician and works directly with patients, said about 3 per cent of people will experience psychosis at some point in their lives and 1 per cent will develop schizophrenia, which is a lifelong condition. A similar large-scale analysis led by Dr Large and published in 2011 probed 83 studies and found the mean age of psychosis onset for cannabis-users was 2.7 years younger than non-users. This new analysis, which looked at 149 international studies involving more than 71,000 people, suggested the age of psychosis onset for cannabis-users was 2.5 years earlier, on average. Dr Large said the similarities between the two suggested the overall conclusion was robust. Jack Wilson, a research fellow at The University of Sydney's Matilda Centre who did not contribute to the study, said it added to "the growing body of evidence" that cannabis use was associated with a greater risk of developing psychosis. "It's certainly providing new evidence on the likelihood of developing psychotic disorders in different age groups, according to whether someone is using cannabis or not," he said. "And that is quite concerning, given the relative ease at which we can now access increasingly potent cannabis products in Australia." Dr Wilson said higher consumption of tetrahydrocannabinol or THC was "associated with the greater risk of harm". One of the study's limitations was that it didn't probe the type of cannabis products that were being used, Dr Wilson said, and extra information about potency would be useful to know. With about 2.5 million Australians using cannabis a year, Dr Large said he was also concerned that cannabis potency was increasing over time, describing some as "amazingly potent". Young people still at risk Despite the study's stark findings about older Australians who use cannabis, Ms Stevens said it was important to note that younger people were certainly not immune to harm. "Everyone, no matter the age, is at risk of some kind of adverse event from using cannabis," she said. Dr Wilson agreed, adding that the accelerated onset of psychosis was just one measure of harm. "There is no safe time to use cannabis, but the most dangerous or the most harmful time to use cannabis is at a younger age, particularly before the age of 25, when we're still seeing neurodevelopment," he said. While more research was needed, Dr Wilson said changes in the developing brain could contribute to the development of psychiatric disorders, such as schizophrenia. If possible, Dr Wilson urged people to avoid more potent products, which had the potential to cause not just psychosis, but greater overall harm. "Delay the onset of use. We know that harm is greatest in those that are using cannabis at a younger age," he said. "We also know that you know that smoking cannabis is associated with cardiovascular risk, so if people can avoid smoking and perhaps avoid mixing with tobacco." Prompting calls for more health warnings Researchers are now calling for greater public health messaging about the use of cannabis regardless of whether it is used recreationally or as a legal medicinal product. They would also like to see stronger regulations regarding the sale of medicinal cannabis in Australia. Medicinal cannabis is regulated in Australia and must be prescribed by a registered healthcare professional. Guidelines suggest it cannot be used as a first-line treatment. Ms Stevens said as a scientist, she wasn't able to provide an in-depth answer about best-practice legislation, but said the way medicinal cannabis was being marketed and sold in Australia was "quite inappropriate". She said some providers selling medicinal cannabis claimed to operate under Therapeutic Goods Administration (TGA) guidelines, claiming to be "fully TGA-approved". But most medicinal cannabis products are classed as unapproved therapeutic goods, which means they have not been assessed for safety, quality or efficacy. Dr Large was more scathing in his assessment. "I personally think that the medicationalisation of [cannabis] has quashed the discussions about the toxicity of cannabis," he said. "It's that part of public policy that I think will quite likely prove to be one of the biggest public health bungles in Australian history."