Tags: cannabis use; psychotic disorder; genetics; age of onset; clinical high risk

 

Thumbnail: Conflicting studies point to various factors that might make it more likely for cannabis users do develop a psychotic disorder, which is different than acute psychosis that can occur occasionally during cannabis use.  This systematic review found that use at an earlier age, more frequent (especially daily) use, and high-potency cannabis increased the risk of psychosis. In addition, people with a previous risk (e.g. due to family history) seem to be especially vulnerable. The reviewers suggest that changing cannabis use habits and behaviors might be an effective strategy to reduce risk.

 

The problem: Increasing evidence suggests there is a relationship between cannabis use and the risk of psychosis, but it is unclear exactly which factors may contribute to this.

 

The study: An extensive literature search of was conducted, using the MEDLINE and Embase databases, to identify relevant studies conforming to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The search included 56 studies published between 2009 and 2019.

 

Main Points:

• In the last decade, 6 studies investigated patterns of cannabis use relative to the development of psychosis symptoms.All found that that frequent cannabis use is associated with an increase in psychotic disorders and psychotic-like experiences. This was especially true when cannabis was used on a daily basis.

• 6 studies looked at the influence of age of first cannabis use on the development of psychosis. While each study had a slightly different age cut off for “early use”, they all agreed that starting cannabis use at a younger age is related to an increased risk of developing psychosis.

• 22 studies examined the association between cannabis use and the age at which psychosis develops. The majority of these studies found that cannabis use is related to an earlier onset of psychotic symptoms, psychotic experiences or psychotic disorders.

• 4 studies reported on the type of cannabis used, in relation to the risk of developing psychosis. The results from these studies suggest that using high-potency cannabis (cannabis with a higher THC content) significantly increases the risk of developing a psychotic disorder.

• 3 studies looked at the relationship of genetic background to the risk for developing psychosis. Each of these studies looked at a different gene. Overall, there is some evidence for an association between some genes involved in dopamine signaling and an increased risk of psychosis. However, a significant number of studies found no association for this.

• 5 studies investigated the relationship between cannabis use and the transition to psychosis, in individuals at a high clinical risk and found that frequent cannabis use, cannabis dependence, or lifetime cannabis abuse increases the risk of transition to psychosis in these individuals.

 

 

Conclusions: Overall, the studies reviewed here show that there is an association between cannabis use and developing psychosis. The strongest evidence links the frequency and potency of cannabis use to a higher risk for psychosis. There is also evidence that the genetic background of an individual can affect this risk, and that earlier use of cannabis is associated with an earlier onset of psychosis.

 

Why this is a good study:

• This study provides a detailed and up-to-date overview of our understanding about how cannabis use is related to psychosis risk.

• The authors eliminated studies that included heavy use of other drugs, which could confound cannabis-specific findings.

 

Why this isn’t a perfect study:

• This relied on previously conducted studies, so each study was conducted under slightly different conditions and had different controls.

• While the authors eliminated studies reporting high levels of other drug use, it is very difficult to exclude all other drug use (e.g.cigarettes or alcohol), so the conclusions reported here may be confounded by the use of substances other than cannabis

• The frequency, type and amount of cannabis used was not uniform across studies, so the conclusions drawn here may apply differently to different individuals, based on their usage habits

• These studies do not take into account potential environmental factors that may be playing a role. This could be particularly relevant to the genetic studies.

 

 

What this study adds:

• This study provides a concise summary of the state of the field

• This confirms that we need more research to clarify which genes are playing a role in this relationship, and the extent to which they are effecting the progression to psychosis.

• This provides support for the theory that modifying cannabis use behavior may be a viable method to reduce the development of psychosis in some individuals

 

What it doesn’t:

• This study does not provide insight into the biological mechanisms that underlay this association, making it difficult to develop preventative measures.

 

Funder:  No agency funding was reported

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Author conflicts:  None