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expert reaction to cannabis use in adolescents and risk of depression, anxiety and suicidality

Reactions to a meta-analysis published in JAMA Psychiatry which demonstrates a correlation between cannabis use and depression/anxiety in young people.

Dr. Joseph Firth, Senior Research Fellow, NICM Health Research Institute at Western Sydney University, said:

“Among young adults worldwide, depression is the leading cause of disability, and suicide is the most common cause of death. By linking cannabis as a contributing factor to both of these huge issues for public health, this latest study provides new insights into importance of reducing adolescent cannabis use.

“However, it is important to consider that not all cannabis is equal. In particular, high-THC strains of cannabis are typically associated with more severe impact on mental health – whereas another component of cannabis (known as ‘CBD’) may even attenuate some of the adverse effects.

“So, further research considering the varying effects of different types of cannabis use is required to gain a full understanding of the interaction between cannabis use and mental health.

“From a public health perspective, the most important next steps is to determine which interventions, education programs, or health policies can actually reduce cannabis use in adolescents. If such schemes actually can produce significant reductions in the incidence of mental illness and suicide among young adults, these efforts could lead to population-scale prevention strategies for tackling the severe mental health issues currently affecting many young adults all over the world.”

Dr Tom Freeman, Lead Director of the Addiction and Mental Health Group (AIM) at the University of Bath, said:

“This important new study used rigorous scientific methods to identify and combine repeated assessments of cannabis use and mental health in over 20,000 people. The findings highlight adolescent cannabis use as a correlate of developing depression and suicide in young adulthood. However, they do not establish if it was the cause. Some of the studies included in the analysis were unable to rule out the role of other possible influences, such as cigarette smoking, other drug use, and the role of peers.

“A key outstanding question is how different levels of cannabis use might influence an individual’s risk of depression, suicide and anxiety. This analysis compared the most frequent level of use (e.g. using every day) with people who didn’t use cannabis at all. Further research is needed to identify the relationship between lower levels of cannabis use and its effects on mental health.”

Dr Lindsey Hines, Sir Henry Wellcome Postdoctoral Fellow, Centre for Academic Mental Health, University of Bristol, said:

“This study has taken the results of multiple studies of teenagers published over the past 15 years, and pulled together the results from those studies to get a clear, good quality estimate of the association between using cannabis as a teenager and mental health in adulthood.

“We know that cannabis use co-occurs with anxiety, depression and self-harm in teenagers, but this research suggests that teenage cannabis use is still related to mental health in later years. However, we don’t know if cannabis use as a teenage is causing these adult mental health problems. It could be that these behaviours are all due to shared underlying risk factors, such as early adversity or genetics. 

“It’s also important to note that these results don’t tell us if the effects are specifically due to cannabis use during teenage years. It may be that people who were smoking cannabis as a teenager have carried on smoking cannabis as adults, which may explain some of the relationship to mental health.

“This research highlights the vital work we still need to do to understand if, and how, smoking cannabis as a teenager can have effects later in life. Amongst teenagers using cannabis the effect of use on education and social relationships, the frequency or heaviness of cannabis use, and the strength of the drug, are all likely to relate to differences in lifetime mental health outcomes.”

Prof Sir Robin Murray, Professor of Psychiatric Research, Institute of Psychiatry Psychology & Neuroscience, King’s College London (IoPPN), said:

“The study is competently done, the conclusions are backed up by solid data and there is no over speculation. The finding that cannabis use is associated with a modest increase in risk of depression and suicide is probably correct.  We have better quality studies concerning psychosis than depression and it does seem that cannabis use has a greater impact on increasing risk of schizophrenia-like psychosis than depression or anxiety. A smaller risk increasing effect on depression than psychosis is still important given that depression is a lot more common than psychosis.

“But it’s important to note the many limitations of this research. Information about the patterns of cannabis use in the original studies being re-examined is not very detailed; for example they do not quantify the amount of cannabis being smoked or what kind of cannabis is being used -we know from studies of psychosis that the risk is much greater with daily use of modern high potency (high THC/Low CBD) cannabis than old fashioned low THC varieties. We also don’t know the extent to which the subjects were using other drugs or tobacco which may be important.

“Where the press release speaks about pre-clinical animal models and the potential mechanism by which THC acts, this is not new information from the study itself and is already well known and documented.”

Prof Celso Arango, President of the European College of Neuropsychopharmacology, said: 

“It has been known for a long time the relationship between cannabis and psychosis (cannabis causes psychosis, use of cannabis decreases age at onset in cases of first psychotic episode, in people with psychotic disorders cannabis use increases risk of relapse and worsens the prognosis). In this case the meta-analytic assessment shows a relationship between using cannabis during adolescence and later development of depression and suicidality. The major problem of this type of study is the difficulty in assessing causality (e.g. that cannabis causes later depression rather than subjects at higher risk to develop depression being also more prone to use cannabis, for instance to cope with subsyndromic symptoms, even though in this study cases with prior depression were excluded). It’s really important to see if it truly is a case of causality between cannabis use and depression as cannabis use is amenable to preventative interventions. It has been shown that primary prevention for cannabis use in adolescence is possible.¥

‘Association of Cannabis Use in Adolescence and Risk of Depression, Anxiety, and Suicidality in Young Adulthood: A Systematic Review and Meta-analysis’ by Gabriella Gobbi et al. was published in JAMA Psychiatry at 16:00 UK time on Wednesday 13th February.

All our previous output on this subject can be seen at this weblink:

Declared interests

Dr Tom Freeman: No conflicts of interest

Dr Lindsey Hines: No declarations of interest

Prof Sir Robin Murray: No conflicts of interest

Prof Celso Arango:  Dr. Arango. has been a consultant to or has received honoraria or grants from Acadia, Ambrosseti, Gedeon Richter, Janssen Cilag, Lundbeck, Merck, Otsuka, Roche, Servier, Shire, Schering Plough, Sumitomo Dainippon Pharma, Sunovion and Takeda.

None others received.

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