A study published in JAMA Psychiatry looks at cannabis use disorder and subsequent risk of depression and bipolar disorder.
Dr Thomas Richardson, Associate Professor of Clinical Psychology at the University of Southampton, and an expert on Bipolar Disorder, said:
“This is an impressive and important piece of research: with a sample size of well over 6 million this is the largest study to date showing a link between cannabis use and the development of depression and Bipolar Disorder. A strength of this research is that they followed people up for many years and took into account factors such as family history of mood disorders. Previous research has shown a link between cannabis use and manic symptoms in the general population, and that cannabis use leads to more severe symptoms in those with a diagnosis of Bipolar disorder. This study shows that those with cannabis use disorder are more than 2.5 times more likely to develop Bipolar disorder.
“It is important to say a few issues with this research though. Firstly the risk of Bipolar disorder is increased, but it is still only a minority of cannabis users who developed Bipolar: 14%. This is very high compared to the general population where we would expect it to be maybe 2%, but it is still a minority of cannabis users. This also used data from hospital treatment records for both mood disorders and cannabis use. So it doesn’t take into account the many people who might be undiagnosed Bipolar. This also means that it’s people with cannabis use disorder who are getting treatment: people who are showing signs of dependence, for example being unable to stop despite it causing problems. This probably only impacts around 10% of those who use cannabis. So this study does not tell us about the risk of depression and bipolar in those who use cannabis occasionally and do not have any major issues with it.
“There is a lot more research about cannabis use and the risk of psychosis. We know that you are more likely to develop psychosis from cannabis use if you are using really strong ‘skunk’, using frequently, and starting use at an early age. We also know that it interacts with other risk factors: for example you are at greater risk of a history of childhood sexual abuse. For this study and Bipolar disorder, we don’t know exactly whether these factors’ strength and frequency of cannabis use have a role to play, and how it interacts with other risk factors. But it is clear that those who have problematic cannabis use are at a greater risk of developing depression and Bipolar disorder. So if you are someone who is prone to depression, or has a family history of Bipolar disorder it is worth being cautious in cannabis use.
“I would like to see further research showing the specific mechanisms of these links: i.e. why and how does cannabis use disorder increase the risk of Bipolar. It is unclear whether and how we can actually stop people using cannabis in the first place. A ‘just say no’ approach is unlike to reduce the prevalence of Bipolar disorder at a population level. A better approach is to try to targeted interventions for those with more severe cannabis use problems and the greatest risk of Bipolar disorder.”
Prof David Curtis, Honorary Professor, UCL Genetics Institute, said:
“I don’t think this study provides strong evidence that using cannabis increases one’s risk of depression or bipolar disorder.
“The study shows that people with depression or bipolar disorder are more likely to have been previously assigned a diagnosis of ‘cannabis use disorder’. But in order to have been registered as having ‘cannabis use disorder’ one needs to have had some kind of psychiatric or emotional problem with impaired functioning in addition to consuming significant quantities of cannabis. So it is entirely possible that the people who get assigned this diagnosis are those who have a susceptibility to depression and related disorders. In fact, it may well be that people with a depressive illness who were also consuming cannabis were sometimes mistakenly diagnosed in the first instance with cannabis use disorder rather than depression.
“Around 20% of young people in Denmark use cannabis but in this study fewer than 1% of the sample had been assigned a diagnosis of cannabis use disorder and these may well have been people who had a pre-existing susceptibility to depression and other psychiatric disorders.
“Other studies have shown that cannabis use itself is not associated with increased risk of depression, so perhaps we are seeing that people with a pre-existing susceptibility to depression are more likely to be diagnosed with cannabis use disorder. This clearly does not mean that we should infer that cannabis somehow causes depression.
“I don’t think that this study provides us with much information to help decide the extent to which cannabis use is, or is not, harmful.”
‘Cannabis Use Disorder and Subsequent Risk of Psychotic and Nonpsychotic Unipolar Depression and Bipolar Disorder’ by Oskar Hougaard Jefsen et al. was published in JAMA Psychiatry at 4pm UK time on Wednesday 24 May 2023.
Prof David Curtis: “I have no conflict of interest to declare.”
Dr Thomas Richardson: “None to declare.”