A major international review has found that people who use cannabis tend to develop psychosis earlier than people who do not use it.
Across the studies examined, the difference was about two and a half years on average.
The gap became much larger among people who developed psychosis later in adulthood. Among those whose psychosis began at age 30 or older, cannabis users experienced it more than six years earlier on average than nonusers.
The research was led by scientists at UNSW Sydney in Australia. They combined results from 149 studies involving more than 71,000 people, making this one of the largest analyses of cannabis use and the timing of psychosis.
Psychosis is a condition in which a person can lose touch with shared reality. It can involve hearing or seeing things that other people do not, strongly believing things that are not true, confused thinking or major changes in behavior.
Psychosis can occur in several mental health conditions, including schizophrenia and some forms of bipolar disorder and severe depression. It can also sometimes be triggered by drugs or other medical conditions.
Most first episodes occur during late adolescence or early adulthood, although psychosis can begin at many different ages. Because the condition can seriously disrupt education, work and relationships, even a difference of a few years in when it begins can matter.
The UNSW researchers examined studies largely conducted in Europe, North America and Australia since 2011. Many of the participants had schizophrenia.
When the average age of psychosis onset in a study was between 20 and 24, cannabis users developed psychosis about 1.6 years earlier than nonusers. The difference became larger as the average age at onset increased.
In studies where psychosis typically appeared between ages 25 and 29, cannabis users developed it about 4.43 years earlier. Among groups where psychosis developed at age 30 or later, the gap reached about 6.3 years.
The researchers did not find a significant difference among teenagers. However, this does not necessarily mean cannabis carries no risk for adolescents.
Psychosis is uncommon before the middle teenage years, leaving little room for cannabis use to shift the first episode to an even younger age. Strong genetic and developmental factors may also have a larger influence on psychosis that appears very early.
Chief investigator Professor Matthew Large, a psychiatrist at UNSW’s School of Clinical Medicine, said the findings suggest that the effects of cannabis exposure may build over time. This challenges the idea that waiting until the mid-20s makes cannabis use free of psychiatric risk.
One possible explanation is that longer exposure produces a cumulative effect. Someone who begins using cannabis and continues for years may experience a different level of risk from someone with brief or occasional exposure.
THC is the main substance in cannabis responsible for the drug’s intoxicating effects. It acts on systems in the brain involved in perception, memory, mood and thinking, and high doses can cause temporary psychotic symptoms in some people.
Scientists have long investigated whether cannabis can contribute to longer-lasting psychotic illness in people who are vulnerable. Earlier research has linked frequent use and stronger cannabis products with greater psychosis risk.
The new study focused on the age at which psychosis begins rather than simply asking whether cannabis users are more likely to develop it. Lead author Carly Stevens said comparing different life stages revealed a clearer pattern than previous analyses could show.
An earlier meta-analysis led by Large in 2011 found that psychosis began about 2.7 years earlier among cannabis users. The much larger new analysis was able to examine how this difference changed with age.
The researchers stressed that their study cannot prove cannabis directly caused psychosis to begin earlier. The evidence came from observational studies rather than experiments in which people were deliberately assigned to use cannabis.
Such an experiment would be unethical because researchers could not knowingly expose people to a substance suspected of increasing their risk of serious mental illness. Scientists therefore have to rely on long-term studies, medical records and other forms of observational evidence.
Other factors may also differ between cannabis users and nonusers. Genetics, use of other drugs, childhood experiences, social conditions and existing mental health symptoms could influence both cannabis use and psychosis risk.
There is also a difficult question about cause and timing. Some people may begin using cannabis partly because they are already experiencing early symptoms of mental illness, even before a first clear psychotic episode occurs.
Despite these uncertainties, the researchers said the consistency of the evidence supports taking the association seriously. The particularly large difference among older groups may fit with the idea that longer cannabis exposure can influence when psychosis appears.
An earlier onset can have important consequences. Developing psychosis during university, vocational training or the early years of a career may interrupt education and employment at a particularly important stage of life.
It can also affect relationships and independence. Effective treatment can help many people, but serious psychotic disorders may require long-term medical and social support.
The researchers believe public health messages should therefore extend beyond teenagers. While protecting adolescents remains important, adults in their 20s and beyond should also understand the possible connection between cannabis and psychosis.
This has become increasingly relevant as cannabis laws change and medical cannabis becomes more common. Some people may assume that a product described as medical or legal is automatically free from serious risks, but the effects can depend on dose, THC strength, frequency of use and individual vulnerability.
Edible cannabis products create additional concerns because their effects can take longer to appear. People may consume more before realizing how strong the dose is, and some products can contain high amounts of THC.
The study was led by researchers from UNSW Medicine, including Professor Matthew Large and lead author Carly Stevens. It was published in the scientific journal Biological Psychiatry.
The study’s major strengths are its large sample and its ability to compare psychosis onset across different ages. However, it cannot prove cause and effect, and the studies did not all measure cannabis exposure in exactly the same way.
Overall, the research strengthens evidence that cannabis use is associated with an earlier first episode of psychosis and suggests that the gap may become larger with longer exposure. The findings do not mean every cannabis user will develop psychosis, but they support clearer warnings for people who may be vulnerable and more research into dose, frequency and THC strength.
Source: UNSW Sydney
