"Teen cannabis use is to blame for 60,000 people suffering depression in the UK," reports The Sun.
A review has found that teenagers aged under 18 who used cannabis were 37% more likely to get depression in early adulthood than teenagers who didn't.
Cannabis use among teenagers has been linked to poor mental health previously, with research finding a strong link between cannabis use and schizophrenia. About 4% of adolescents aged 11 to 15 in England are thought to use cannabis each month. That means a lot of people may be at increased risk of mental health problems.
This study cannot prove that cannabis use in teenagers caused depression in young adults. Depression is a complex disorder with many potential risk factors, including hereditary influence and life circumstances. Other, unmeasured factors may have contributed to the study results. For example, difficult childhood circumstances could increase both the chances of getting depression as a young adult, and of using cannabis as a teenager.
However, the study joins other research to suggest that cannabis use may be putting teenagers at risk of poor mental health.
Researchers make the point that as teenagers' brains are still developing up until adulthood, exposure to cannabis could impact on this development in a number of potentially harmful ways.
Read more facts about cannabis.
The researchers who carried out the analysis were from McGill University in Canada, Rutgers University in the US and the University of Oxford in the UK. It was funded by the Canadian Institutes of Health Research, the Quebec Network on Suicide, Mood Disorders and Related Disorder, and the UK's National Institute for Health Research. The study was published in the peer-reviewed journal JAMA Psychiatry.
The research was widely reported in the UK media. The headline claims in the Mail Online and elsewhere that "half a million" cases of adult depression "could be prevented" if teenagers avoided cannabis are an oversimplification. They are based on the researcher's estimate that 7% of the cases of depression recorded in the study among adults aged 18 to 32 might be attributable to cannabis use. It is unclear whether this statistic would translate to all adult cases of depression in the UK.
The reporting in The Guardian and BBC News did explain that the results could not prove a direct cause and effect link.
Other UK sources did not make this point clear.
This was a systematic review and meta-analysis of cohort studies. Systematic reviews and meta-analyses are good ways of summarising the state of research into a topic. Cohort studies are good for spotting patterns and links between risk factors. However, they cannot prove cannabis use causes depression. Other, unmeasured confounding factors might affect the results.
The researchers looked for cohort studies that had recorded cannabis use of people aged under 18, and then followed them up to see if they developed anxiety, depression, suicidal thoughts or attempted suicide, up to the age of 32. The studies had records of whether people already had anxiety, depression or suicidal thoughts at the start of the study and adjusted their figures to take account of this.
Researchers looked to see whether under-18s who had used cannabis were more likely to develop these mental health conditions between the ages of 18 and 32, compared to non-users, and pooled the data from the studies. The reports of cannabis use were based on self-reported questionnaires filled in by the adolescents. Depression, anxiety and suicidal thoughts were assessed by a variety of questionnaires, interviews and symptom scores.
The researchers checked the studies for potential sources of bias and conducted an analysis to see whether 2 studies that might have had overlapping populations had a strong influence on the results (they concluded this made little difference to the overall result).
The researchers found 11 studies of interest, with 7 studies looking at depression, 3 at anxiety and 3 at suicidal thoughts or attempts (some studies looked at more than 1 outcome). The total number of people studied was 23,317, although it's not reported how many people were involved in the analysis of each outcome.
The chance of having suicidal thoughts was 50% higher (OR 1.50, 95% CI 1.11 to 2.03) and the chance of making a suicide attempt was almost 4 times higher (OR 3.46, 95% CI 1.53 to 7.84).
There was no increased risk of anxiety in early adulthood linked to cannabis use in adolescence.
The researchers said: "This meta-analysis suggests that cannabis exposure could be one factor contributing to depression in young adulthood." They described the risk of depression as "moderately increased" but acknowledge that a "strong causal association cannot be made".
However, they say that the research suggests "cannabis is a serious public health concern and there is an urgent need to implement better drug use prevention programs targeting the use of cannabis among adolescents".
This study raises the possibility that cannabis, the most commonly used illegal drug in the UK, could increase the risk of depression and suicidal thoughts or suicide attempts. We already know its use seems to be linked to the less common condition of schizophrenia.
That's not to say that it definitely causes depression or other mental health problems. There are a number of limitations to the research.
This type of study cannot prove that one factor directly causes another, as there are many factors that can influence both the likelihood of getting depression and likelihood of using cannabis.
The study doesn't include some important information, such as the amount or potency of cannabis used, whether people used other drugs, their alcohol and tobacco use, or whether people were in school and what their home environment was like. We don't know whether people in the studies continued to use cannabis after the age of 18, so continuing drug use could also have affected the results.
The studies included in the review assessed depression and suicidal thoughts through a variety of methods, so we don't know if the results would have been the same if all studies had used the same method.
While the study does provide the increase of risk in depression in people who smoked cannabis compared to those who didn't (the relative risk), it doesn't provide a figure of how many people were in this group. And this group could consist of just a small amount of people. So the overall increase in risk in terms of population levels (absolute risk) could still remain low.
Having said that, this is one of those cases where it would seem sensible to avoid the risk of cannabis if possible, especially if you are a young person who's brain is still developing.