February 17, 2016

expert reaction to US study looking at cannabis use and risk of substance use disorders

Publishing in JAMA Psychiatry a group of scientists have examined the mental health effects of cannabis use and report that within the population in which they looked cannabis use was associated with increased risk for substance abuse disorders in the following years, but was not associated with any increased risk of depression anxiety.

Dr Amir Englund, Post-doctoral researcher in Psychopharmacology at the Institute of Psychiatry, Psychology and Neuroscience (IoPPN), King’s College London, said:

“The present study is a large study exploring the effects of cannabis use on future problems such as anxiety, depression and drug and alcohol addiction. They followed close to 35 thousand participants for three years between 2001-2002 and 2004-2005. They found that use of cannabis was related to increased risk of later addiction to alcohol, cannabis and other drugs. Cannabis was not related to anxiety or depression at follow-up.

“The researchers controlled for a host of psycho-social factors, which greatly reduced the strength of these associations – this suggests that psycho-social factors (things like history of drug problems, parental loss (divorce or death), poor family environment, low self-esteem, social deviance, personality disorders) play an important role in mood and addictive disorders.

“Of course a study such as this is unable to ascertain causality between cannabis use and later drug addiction, merely that a relationship exists. The authors also report a dose-response effect where greater use of cannabis is related to a greater risk of later addiction. However, they use a crude measure of frequency of cannabis use (less than monthly or at least monthly) and there was no information as to the potency of the cannabis the participants used which we know from previous studies¹ is related to the addictiveness of cannabis.

“Towards the end of the article the authors make comments that go beyond what their data here show, when claiming their results hold relevance for policy decisions regarding cannabis use. The effect of legislative change on risk of addiction is not something the researchers explored in this study and so they cannot draw conclusions on it from the data in this study. A study² published just this week found that cannabis use problems have remained at a stable level between 2002 and 2013. Furthermore, some of the co-authors of this paper published a study³ last year which found no increase in cannabis use among adolescents following changes to medical cannabis laws. It is important to distinguish between risk of cannabis use and risks of legislative change, as they are not the same thing.”

1 http://www.ncbi.nlm.nih.gov/pubmed/26213314

2 http://archpsyc.jamanetwork.com/article.aspx?articleid=2491353

3 http://www.thelancet.com/journals/lanpsy/article/PIIS2215-0366%2815%2900217-5/abstract

Claire Mokrysz, PhD student in the Department of Clinical, Educational and Health Psychology, UCL, and author of a recent longitudinal study on cannabis use outcomes, said:

“This large representative sample from the USA demonstrates robust associations between cannabis use and various substance use disorders three years later – including alcohol, nicotine and cannabis addictions. While interesting this finding is not unexpected – many people with a substance use disorder are poly-drug users and since cannabis is the most commonly used illicit drug across the world, it is unsurprising that past cannabis use is associated with future substance use disorder.

“Since the authors looked only at cannabis use we cannot know whether this association is specific to cannabis or whether similar results would be found if past use of other substances were to be explored.

“More interesting is that they found no evidence of past cannabis use leading to depression or anxiety – suggesting that increased rates of depression and anxiety in cannabis users may be explained not by their cannabis use but by other environmental or genetic factors.”

‘Cannabis use and risk of psychiatric disorders: prospective evidence from a US national longitudinal study’ by Carlos Blanco et al. published in JAMA Psychiatry on February 2016. 

Declared interests

Dr Amir Englund: “I am employed by King’s College London and have no relevant financial or other conflicts of interest.”

Claire Mokrysz: “I’m funded on an UK MRC Studentship and am a student member of the British Association of Psychopharmacology (BAP) and the Society for the Study of Addiction (SSA).”