Cannabis use is linked to significantly earlier onset of psychosis, and age is a key factor – found new research from the University of New South Wales.
Despite the vulnerability of the adolescent brain, the research suggests that cannabis may play a more influential role in the timing of psychosis onset in older people, especially beyond the age of 25, said lead author Carly Stevens, from UNSW’s School of Biomedical Sciences.
‘The link between cannabis use and accelerated psychosis seems the most severe in people who tend to develop psychosis aged 30 or over,’ she said.
Association with earlier psychosis onset increases with age
The meta-analysis found that cannabis use was linked to earlier onset of psychosis in older people. Cannabis users who developed psychosis in their early twenties did so 1.6 years earlier than non-users on average, rising to 4.43 years earlier among those in their late twenties, and 6.3 years earlier among those aged 30 and over.
Ms Stevens said the findings should put to rest a common assumption that the harms are most impactful for a developing brain.
‘There is this widespread belief that the brain matures at approximately 25 years old (which is now being contested by more recent research) and that using cannabis after this age is less harmful; this myth is so prevalent that I’ve even had a doctor tell me they waited to try cannabis until after that age because they thought they’d be safe from long-term health problems,’ she said.
‘What we’re seeing suggests a cumulative effect, where harm builds up over time, a bit like alcohol or nicotine,’ added Chief investigator and psychiatrist Matthew Large, a Conjoint Professor with UNSW’s School of Clinical Medicine.
Ms Stevens said it is ‘well-established’ that adolescent cannabis use puts people at risk of a range of serious psychiatric and cognitive consequences, including life-long psychotic disorders and memory impairment.
‘However, [because] our research suggests that older people may be more at risk of accelerated psychosis, it’s crucial that pharmacists educate patients that they risk experiencing serious health-related harms if they use cannabis at any age.’
More harmful than helpful?
Legalising medicinal cannabis has perpetuated the idea that cannabis can’t be that harmful to a person’s health, Ms Stevens said. ‘This is understandable given the trust people put in healthcare professionals to not put them at risk of serious harm,’ she said.
But Prof Large has treated a growing number of people with psychosis who had been prescribed cannabis for conditions where the evidence of benefit was limited.
‘It’s got this therapeutic halo around it, which blinds people to the risk,’ he said.
Mental health conditions are among the most common reasons medicinal cannabis is prescribed in Australia. Between 2016 and 2022, a third (33.9%) of medicinal cannabis Special Access Scheme Category B applications were for psychiatric conditions.
Yet a recent systematic review found no evidence that medicinal cannabis is effective for anxiety, depression or PTSD, and TGA guidance states THC-containing products are ‘generally not appropriate’ for patients with psychotic or active mood and anxiety disorders.
But the scale of prescribing has grown enormously. The number of patients on unapproved medicinal cannabis products rose from around 18,000 in 2019 to more than one million by early 2024.
Ms Stevens said pharmacists could help to warn patients that most cannabis products containing THC are not TGA-approved, meaning they have not been adequately assessed for safety and efficacy.
‘Further, they could counsel patients on some of the more severe health problems associated with cannabis, including higher risk of schizophrenia and dementia, to help dispel this myth that cannabis is accessible medically and therefore safe.’
Identifying risks at the counter
Medicinal cannabis products are often prescribed through ‘closed loop’ models – where a patient’s medical history may not be understood, nor their prescribing history entered into real-time prescription monitoring systems.
That closed-loop gap is where Ms Stevens sees the greatest opportunity for pharmacists to intervene to catch patients at risk of developing psychosis.
‘The most effective way would be to ask patients directly if anyone in their family, including extended family, has ever experienced psychosis, even if it was a single episode, or lives with a lifelong disorder like schizophrenia,’ she said.
For patients unfamiliar with clinical terminology, she suggested a more accessible approach: ‘Has anyone in your family ever heard voices or seen things that aren’t there?’
‘This approach can also be used to assess individual risk, as people who have experienced even subthreshold psychotic symptoms are more vulnerable to developing psychosis,’ she added.
