Scientists believe that they have identified around 60,000 cases of depression among adults under the age of 35 in the UK and more than 400,000 in the US, which could be avoided if adolescents do not smoke cannabis.
An international team of scientists examined eleven studies published since the mid-1990s involving a total of more than 23,000 people, they report in the journal JAMA Psychiatry. They investigated the use of cannabis for non-medical purposes in children under the age of 18 years. The participants were then followed into adulthood to see who was suffering from depression, anxiety or suicidality. Not a single study addressed all three mental health problems.
Taking into account factors such as age, initial mental health problems and socioeconomic status, cannabis use was associated with the likelihood that later clinical depression, suicidal thoughts or suicide attempt would occur. The likelihood of attempting suicide was nearly 3.5 times higher among those who consumed cannabis before the age of 18, compared to those who did not – although the authors described the number as inaccurate.
Dr. Andrea Cipriani, co-author of the University of Oxford study, said: "The number of people exposed to cannabis, especially at this vulnerable age, is very high, and I think that should be a priority for public health and health also in the medical field. "
The most important psychoactive ingredient in cannabis is THC or delta-9-tetrahydrocannabinol, which binds to receptors in the brain that are particularly prevalent in regions that are important for emotions, learning, and rational thinking. The density of such receptors in these regions reaches a peak during adolescence. Along with the fact that the brains of young people are still evolving, this has led to growing concern about the effects of cannabis on young consumers, Cipriani said.
The link to depression may be due to the presence of receptors that bind THC in parts of the brain where the neurotransmitters serotonin and norepinephrine were produced.
According to recent figures, more than 17 million young people between the ages of 15 and 34 in the EU and 14.1% have used cannabis in the past year, although there are large differences between countries. In England and Wales, around 11.5% of people in this age group said they had used cannabis the year before, while 2011 data show that about 4% of 14- to 19-year-olds in England use cannabis in a given month.
While the authors of the study say the apparent effects are of modest size, they emphasize that the effects are by no means negligible. Applying the results to the known prevalence of clinical depression in adults under 35 years of age concludes that one out of 14 cases in this age group could be avoided if adolescents did not take the drug.
The team says that greater education is needed both for parents who are not likely to harm their children with cannabis, and for teenagers themselves, and that emphasis should be placed on adolescents not taking the drug.
"Now we know that there can be a biological effect, especially among teenagers, whose consequences can be devastating," said Cipriani.
They said the issues of decriminalization and legalization of cannabis are difficult. One argument in favor of legalization is that adults could buy the drug in stores and age it, thereby eliminating illegal traffickers and stopping the source of cannabis for under-18s. It was also argued that legalization would allow consumers to choose the strength of the cannabis they use.
While the latest research results ring out a number of animal studies on the effects of cannabis on depression, there are some limitations. Statistics on cannabis use were self-reported, with different frequency of use between studies. The researchers could not rule out that the results could reflect a more complex situation than the simple "concern and impact" of cannabis on mental health problems. In other words, other factors could play a role in the linkage. Moreover, it is not clear that only cannabis is important in puberty. It may also be important to use until adulthood.
Sir Robin Murray, professor of psychiatric research at King's College London, who was not involved in the research, said it was important to note that the studies studied did not examine whether participants were using other drugs or how much Cannabis or what kind of cannabis there were users had taken. In fact, the studies began before highly potent cannabis became commonplace.
"From studies on psychosis, we know that the risk of daily use of modern high-potency cannabis is much higher than that of old-fashioned low-THC varieties," he said.
Dr. Tom Freeman, executive director of the Addiction and Mental Health group at the University of Bath, said that further research is needed to inform consumers about the risks of different products and their level of use.
"We know that exposure to higher THC levels is associated with an increased risk of adverse effects, while another component of cannabis (cannabidiol) may provide protection and may be beneficial to mental health," he said.
• In the UK, Samaritans can be contacted at 116 123 or by e-mail at email@example.com. In the US, the lifecycle of National Suicide Prevention is 1-800-273-8255. In Australia, the Lifeline Crisis Support Service 13 11 14. For more international suicide helplines, visit www.befrienders.org.