Many studies of the relationship between cannabis use and mental illnesses such as depression and anxiety have suffered from methodological issues by not controlling for related factors.
The few longitudinal studies that have been conducted have mixed findings.
A 2014 review of the existing research concluded that using cannabis placed an individual at moderate risk of developing depression.
Unfortunately it was not within the scope of the research to determine if cannabis use was causing depression or if the relationship instead reflects the association between cannabis use and social problems. Cannabis use is associated with other factors that increase risk of depression such as school dropout and unemployment.
The relationship between cannabis use and anxiety is also complex. Many people use cannabis for its euphoric and relaxing effects. But some people also experience feelings of anxiety or paranoia when intoxicated. As such, cannabis could be used to relieve anxiety or stress for some while causing others to feel anxious.
A 2014 review of the available research concluded that using cannabis placed an individual at a small risk of developing anxiety. But the authors noted that while the weight of evidence supported the coexistence of cannabis use and anxiety, there was relatively little evidence to suggest that cannabis caused anxiety.
Not included in these previous reviews of depression and anxiety disorders were two recent investigations of cannabis use in the United States using data from 2001-2002 and 2004-2005. These included a host of variables such as demographic status and family environment.
Each found a significant association between cannabis use and the onset of depression and anxiety disorders. But this association was no longer significant when considering the impact of the included variables.
Clearly, the relationship between cannabis use and depression and anxiety disorders is complex and involves the individual's reasons for cannabis use and external situations. That is, cannabis may be used to help cope with social problems that were not necessarily caused by cannabis use.
Schizophrenia
In contrast, the relationship between cannabis use and risk of developing symptoms of psychosis has been well established in many different review articles.
This research has found that early and frequent cannabis use is a component cause of psychosis, which interacts with other risk factors such as family history of psychosis, history of childhood abuse and expression of the COMT and AKT1 genes. These interactions make it difficult to determine the exact role of cannabis use in causing psychosis that may not have otherwise occurred.
Regardless, the connection between cannabis use and psychosis is not surprising. There is a strong resemblance between the acute and transient effects of cannabis use and symptoms of psychosis, including impaired memory, cognition and processing of external stimuli. This combines to make it hard for a person to learn and remember new things but can also extend to the experience of deluded thinking and hallucinations.
We also know that cannabis use by people with established psychotic disorder can exacerbate symptoms.
Overall, the evidence suggests cannabis use will bring forward diagnosis of psychosis by an average of 2.7 years.
The risk of developing schizophrenia increases with the duration and dose of cannabis use. Regular cannabis users have double the risk of non-users. Those who have used cannabis at some point in their life have a 40% increased risk compared with non-users.
That said, it is important to view this increased risk in context. The proportions of individuals with psychosis among the population and among cannabis users are low. Current estimates suggest that if frequent long-term cannabis use was known to cause psychosis, the rates of incidence would increase from seven in 1,000 in non-users to 14 in 1,000 cannabis users.
