Waiting until adulthood to use cannabis may not remove its possible mental health risks, according to a large new study.
Researchers found that cannabis users who developed psychosis tended to experience their first episode years earlier than people who did not use cannabis.
The difference was relatively small among people whose psychosis appeared in their early 20s but grew sharply in older groups. For people who developed psychosis at age 30 or later, cannabis users reached that point about 6.3 years earlier on average.
Researchers at UNSW Sydney reached the finding after reviewing 149 studies involving more than 71,000 people. The global analysis was based mainly on research from Europe, North America and Australia.
Psychosis can dramatically change how a person experiences the world. Someone may hear voices, see things that are not there, develop powerful false beliefs or have thoughts and speech that become difficult for others to follow.
It is not a diagnosis by itself in every case. Psychosis can be a feature of schizophrenia and other mental illnesses, and it can also occur in connection with drugs or certain physical illnesses.
A first psychotic episode commonly occurs during late adolescence or young adulthood. This is often a period when people are finishing education, beginning careers, forming relationships and becoming more independent.
For that reason, moving the onset of a serious illness forward by even two or three years can have a large effect on a person’s life. Earlier illness may interrupt study, work experience and the development of social and financial independence.
The UNSW team wanted to understand whether cannabis use was linked not only to psychosis risk but also to the timing of its onset. They found an overall difference of roughly two and a half years between users and nonusers.
But the average hid a much more interesting age pattern. In studies where psychosis generally began between ages 20 and 24, cannabis users experienced their first episode about 1.6 years earlier.
When the typical onset was between 25 and 29, the difference grew to 4.43 years. In groups where psychosis appeared at age 30 or older, cannabis users developed it about 6.3 years earlier.
No significant difference was detected among teenagers. The researchers cautioned against interpreting this as evidence that cannabis is safe for adolescents.
Psychosis rarely begins before the middle teenage years, so there may simply be little room for the age of onset to move any earlier. In people who develop psychosis very young, strong genetic or developmental influences may also dominate.
The increasing difference with age led researchers to consider whether cannabis exposure could have effects that accumulate over time. In other words, years of use may matter in ways that cannot be seen by looking only at whether someone has ever tried cannabis.
Cannabis contains many chemical substances, but THC produces most of the familiar high. THC can alter perception, memory and thinking, and large amounts can sometimes cause paranoia or short-lived psychotic experiences.
Not all cannabis products contain the same amount of THC. Modern high-strength products and concentrated forms can expose users to much larger doses than some cannabis used in previous decades.
Frequency matters as well. A person who uses cannabis every day may have a very different exposure from someone who uses it a few times a year.
Researchers have previously reported that frequent cannabis use, especially use of stronger products, is associated with a greater likelihood of psychotic disorders. The new analysis adds another piece to that picture by concentrating on when the illness first appears.
The study builds on work published in 2011, when a smaller analysis led by UNSW psychiatrist Professor Matthew Large found psychosis began about 2.7 years earlier among cannabis users. At that time, researchers did not have enough information to examine the pattern across age groups.
The much larger collection of studies now allowed lead author Carly Stevens and colleagues to compare different stages of adulthood. The increasing gap was one of the most important findings.
However, the researchers cannot say with certainty that cannabis caused the earlier onset. The studies observed real-world behavior rather than randomly assigning people to cannabis use or nonuse.
Randomly giving cannabis to people for years to see whether they develop psychosis would be unethical. This means scientists have to build the case from several types of evidence rather than relying on the kind of controlled experiment used to test many medicines.
There are other possible explanations that must be considered. People who use cannabis may differ from nonusers in their genetics, social experiences, use of alcohol or other drugs, and previous mental health difficulties.
Another complication is that the earliest stages of psychosis can begin before the first obvious episode. A person experiencing anxiety, sleep problems or subtle changes in thinking might start using cannabis in an attempt to feel better.
This makes it difficult to establish exactly which came first. Better long-term studies that carefully record when cannabis use starts, how often it occurs and when symptoms emerge could help answer this question.
Even with these limitations, the researchers believe the pattern supports stronger public health warnings. Their concern is that messages have often focused heavily on teenagers while giving adults the impression that starting later carries little psychiatric risk.
The findings suggest the issue may be more complicated. The growing difference among older groups is consistent with the possibility that longer exposure could matter.
Changing cannabis markets add urgency to the question. Medical cannabis has become more widely available, while legalization in some places has increased access to recreational products.
Legal or medically prescribed cannabis is not automatically harmless. Risks may vary depending on the reason for use, a person’s medical history, the dose, the amount of THC and how frequently the product is taken.
Edibles may be particularly difficult to judge because the effects can be delayed. Someone may take another dose before the first one has fully taken effect, resulting in unexpectedly high exposure.
The researchers say better studies are needed to measure cannabis exposure in a consistent way. Future work should examine the amount used, frequency, years of use, THC strength and the effects on older users.
The new meta-analysis was conducted by researchers at UNSW Sydney, including lead author Carly Stevens and chief investigator Professor Matthew Large. The findings were published in Biological Psychiatry.
The analysis is persuasive because it brings together more than 71,000 people from 149 studies and finds a clear age-related pattern. Yet its observational design means it cannot prove that cannabis is responsible for bringing psychosis forward by a specific number of years in an individual.
The results also should not be interpreted as meaning that most cannabis users will develop schizophrenia or another psychotic disorder. Psychosis remains influenced by many factors, and individual vulnerability varies greatly.
The most useful conclusion is that cannabis should not be treated as risk-free simply because someone has passed adolescence. For people with a personal or family vulnerability to psychosis in particular, understanding the possible psychiatric effects of cannabis may be important when making decisions about its use.
Source: UNSW Sydney.
