"Teenagers who smoke cannabis have 'poor memory and abnormal brain structures'," the Daily Mail reports. The headline accurately reflects the results of a small study looking at the effect of cannabis use on brain structure.
Researchers compared the results of brain scans and tests of working memory from people who once had a "cannabis use disorder" with those from people with no history of cannabis use, who the researchers deemed "clean".
These results were also compared with those of "clean" people with schizophrenia and people with schizophrenia who also once had a cannabis use disorder.
People who had used cannabis in the past had differences in the shape of certain parts of the brain compared with those who hadn't. These differences were similar for people both with and without schizophrenia.
The researchers found that certain differences in brain shape seen in cannabis users were associated with people's working memory scores and the age that they started using cannabis.
However, because this study only looked at brain scans and memory tests at one point in time, we can't tell whether the brain changes were present before they started using cannabis.
The researchers now want to explore whether people who used cannabis have long-term changes in their brains.
It is illegal to possess or sell cannabis.
The study was carried out by researchers from Northwestern University Feinberg School of Medicine and Washington University in the US.
It was funded by the US National Institute of Mental Health, National Institute on Drug Abuse, Office of National Drug Control Policy and the Warren Wright Adolescent Center at Northwestern Memorial Hospital Stone Institute of Psychiatry.
The study was published in the peer-reviewed journal Schizophrenia Bulletin.
The results of the study were covered by the Daily Mail, whose coverage was extremely good. The headline of the online version of the article ("Teenagers who smoke cannabis have 'poor memory and abnormal brain structures' ") did not imply that cannabis use was responsible for the changes seen.
In both the print and online versions of the article, it was pointed out early on that, "The abnormalities could have existed before they used cannabis". However, it wasn't clear from the article that people with cannabis use disorder were being analysed.
This was a cross-sectional study. It examined whether a "remote" cannabis use disorder (history of cannabis use or dependence, but not in the previous six months) was associated with differences in brain morphology (structure) in people with or without schizophrenia. It also looked at how these differences related to differences in working memory.
The researchers wanted to test several hypotheses:
A cross-sectional study can characterise differences between different populations. However, it can't tell us whether cannabis use caused the differences in brain structure seen.
In this study, cannabis use disorder was defined as cannabis use resulting in three or more of the following in a 12-month period:
Or one or more of the following:
The researchers imaged the brains of:
Participants were matched on age, gender, hand dominance and parental socioeconomic status. They also performed a series of tests to assess working memory.
The researchers looked at a region of the brain called the subcortex. They found that there were significant differences between healthy "clean" people and people with a remote cannabis use disorder in the surface shape of parts of the subcortex. Likewise, there were significant differences between "clean" people with schizophrenia and people with schizophrenia who had a remote cannabis use disorder.
There were no significant differences between healthy "clean" people and "clean" people with schizophrenia. There also weren't any significant differences between people with a remote cannabis use disorder and people with both a remote cannabis use disorder and schizophrenia.
After controlling for nicotine use and antipsychotic treatment, there was no significant difference in working memory scores between healthy people with or without a cannabis use disorder. However, "clean" people with schizophrenia scored significantly higher than people with both schizophrenia and a cannabis use disorder.
The researchers found that certain differences in brain shape seen in cannabis users were associated with working memory scores and with the age that cannabis use started.
The researchers conclude that, "A remote cannabis use disorder may be associated with differences in working memory-related subcortical morphology in both control and schizophrenia subjects."
They go on to say that, "Future longitudinal studies could help determine whether cannabis use contributes to these observed shape differences or whether they are biomarkers of a vulnerability to the effects of cannabis that predate its misuse."
This small study found that people with a past history of cannabis use disorder had differences in the shape of certain parts of the brain compared with people who had never used the drug, and these differences were similar for people both with and without schizophrenia.
The researchers found that certain differences in brain shape seen in former cannabis users were associated with working memory scores, as well as with the age that they started using cannabis.
However, the major limitation of this study is its cross-sectional design, as we can't tell from this study whether the brain changes were present before cannabis use started. The study also includes a fairly small sample of people with and without schizophrenia.
As the researchers say, further research needs to follow people up long term to see if cannabis use leads to brain changes.
Cannabis is a class B drug that is illegal to possess or sell. It can have variable immediate effects on thinking processes and feelings.
People with a personal or family history of mental health conditions such as schizophrenia may be at an increased risk of cannabis having a detrimental effect. The long-term effects cannabis could have on the brain are poorly understood.