Cannabis link to depression, anxiety and suicide – Expert Reaction

A new review investigates the link between cannabis use during adolescence and the risk of developing major depression, anxiety and suicidal behaviour.

While individual risk is moderate to low, the high prevalence of young people consuming cannabis means there could be large numbers of young people affected.

The SMC asked experts to comment on the study, which is available open access on the JAMA website.

Associate Professor Giles Newton-Howes, Department of Psychological Medicine, University of Otago Wellington, Member of the Medical Cannabis Research Collaboration (MCRC) comments:

“This is an important and interesting review of the literature examining the association between teenage use of cannabis and later depression and anxiety. It is a high-quality study, with careful and systematic collection of data, a sensible strategy to examine the data and well-considered comments on research that is related but could not be statistically analysed.

“It is important to New Zealanders as we are considering increasing the number of people who can smoke cannabis for medical reasons (a change that seems to have increased adolescent use overseas) and are going to consider the topic as a nation next year.  It is also important as the study includes New Zealanders in the people studied, and the other included people are from similar social backgrounds, so this study relates to us.

“What the researchers find is a clear, and quite big, association between teenage cannabis use and adult depression, but not anxiety. This association is also found for suicidal thinking and attempts and this association is also quite big, however, there are fewer studied in this part of the analysis. Although this review and analysis does not prove causality, it is further evidence that cannabis has the potential to harm the developing teenage brain, and we ignore these risks at our peril as a nation as we grapple with the cannabis debate. Science is not the only issue to consider, but provides us with a strong source of knowledge to help us in our thinking.”

Conflict of interest statement: I have written on this subject and my views are in the literature. I am an addictions psychiatrist. 

Associate professor Joe Boden, deputy director of the Christchurch Health and Development Study, University of Otago, comments:

“This review both confirms and reinforces findings from the research literature on the adverse psychosocial effects of regular cannabis use by mid- to late-adolescence (15-17 years of age).  This group of cannabis users represents approximately 5 per cent to 10 per cent of the adolescent population, with these individuals being at significantly increased risk of co-occurring mental health and substance use problems, as well as engaging in anti-social behaviour.  Furthermore, individuals in this group will also display higher levels of risk-taking in general, and are more likely to leave school early.

“The findings of this study further reinforce our concerns about the public health implications of any changes we may choose to make to cannabis laws in New Zealand.  Prior studies have shown that cannabis prohibition has not impeded young people from obtaining and using cannabis. The study by Gobbi and colleagues emphasizes the fact that adolescents are a particularly vulnerable group in terms of cannabis consumption, and any changes we make to the laws need to be both: a) designed in such a way as to keep cannabis out of the hands of adolescents; and b) thoroughly evaluated after implementation to ensure that we are not increasing the amount of cannabis-related harm in our society.”

No conflict of interest declared.

Our friends at the Science Media Centre in the UK also have gathered the following expert comment:

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.