The latest edition of the prestigious magazine The British Medical Journal includes an interesting study entitled “Continued Cannabis use and risk of incidence and persistence of psychotic symptoms: 10-year follow-up cohort study." A lot has been said and written about the relationship between cannabis and psychosis. In this article, we will analyse the content of this latest study, as well as its contribution to this controversial subject.
The latest edition of the prestigious magazine The British Medical Journal includes an interesting study entitled “Continued Cannabis use and risk of incidence and persistence of psychotic symptoms: 10-year follow-up cohort study.” A lot has been said and written about the relationship between cannabis and psychosis. In this article, we will analyse the content of this latest study, as well as its contribution to this controversial subject.
An in-depth, 10-year study
What makes this study so interesting is the fact that it is a 10-year follow-up, which understandably boosts the credibility of its results. It is not a study of the experience of a group of individuals at a given moment in their lives, but rather of the variables over the course of several years of these peoples’ lives, giving us a much clearer and more balanced picture of their experience.
The main objective of this study was, on the one hand, to determine if cannabis use in adolescence increases the risk of developing psychotic symptoms, and on the other hand, evaluate the incidence and persistence of subclinical expressions of psychosis in the general population. ‘Subclinical expressions’ refers to a range of psychotic symptoms below the level required for a full clinical diagnosis.
A total of 1,923 German individuals, between the ages of 14 and 24 and classed as belonging to the general population, took part in the study. Initially, 2,210 individuals were included, but a loss of data on consumption and symptoms reduced their number by 287.
Cannabis and psychotic symptoms
The use of cannabis and the presence of psychotic symptoms were analysed at the start of the study, and then at 1.6, 3.5 and 8.4 years from the start of the study, which lasted 10 years in total. The Munich Composite International Diagnostic Interview (M-CIDI) was used. Such interviews cover symptoms, syndromes and diagnoses of several mental disorders in accordance with the definitions and criteria of the ICD-10 (International Classification of Diseases, version 10) and the DSM-IV (Diagnostic and Statistical Manual of Mental Disorders, fourth edition), as well as information on the frequency, duration and intensity of the symptoms, and psycho-social development.
The most important symptoms to assess include persecutory delusions, delusions of thought interference, auditory hallucinations and passivity phenomena.
Psychologists collaborating in the study instructed the participants to read a list of symptoms and asked them if they had ever had these types of experiences and, if so, how severe they were. The answers were classified as either present or absent symptoms.
Different consumption patterns
To establish whether the participants had been exposed to the effects of cannabis, they were asked whether they had used cannabis five or more times in their lives.
To establish the pattern of continued consumption, a 1 to 5-point scale was used, where 1 means “consumption less than once a month” and 5 means “almost every day”.
To determine cannabis consumption between the first and second periods, participants were asked how many times they had used cannabis since the previous interview.
The results of the study indicated that individuals who had never suffered from psychotic symptoms and who had never used cannabis showed an increased risk of developing these symptoms in the period from 3.5 to 8.4 years after the start of the study, if they began consuming cannabis in the period between the start of the study and 3.5 years thereafter. In addition, the continued use of cannabis during the first period increased the risk of suffering persistent psychotic symptoms during the second period.
On the other hand, the appearance of psychotic symptoms throughout the first period was 31% in users, versus 20% in non-users. Throughout the second period, these values were 14% in users and 8% in non-users.
This association between psychotic episodes and cannabis consumption is independent of factors such as age, sex, socioeconomic status, the use of other drugs, urban or rural development, and childhood trauma. An additional adjustment, which took into account the presence of other psychiatric disorders, also had no effect on the results.
The researchers conclude that cannabis use is a risk factor for developing psychotic symptoms, and that the continued use of cannabis may increase the risk of experiencing a psychotic disorder, due to the impact of this consumption on the persistence of symptoms, which are normally transitory in young individuals and are part of their psycho-social development.
As with all studies, this one has some limitations that I think are important to discuss. The first limitation concerns the fact that we are dealing with a population within a specific nationality. If within the German population it seems that cannabis use is a factor in the subsequent development of psychotic symptoms, the same does not hold true for countries where cannabis consumption is not illegal, such as Morocco or Jamaica, where epidemiological studies did not find a greater prevalence of psychotic pathology than in other countries.
Every country has its own laws, customs and drug consumption patterns and precisely for this reason, the results cannot be extrapolated to other countries, at least not with any scientific rigour. As an example, let’s look at Spain’s current situation, where the proliferation of cannabis social clubs will very likely mean a reduction in stress factors which could trigger the appearance of these psychotic symptoms.
We must not forget that it is during adolescence when self-limiting psychotic symptoms are most common and that these are important and necessary for the psychological development of individuals. This study demonstrates that the continued consumption of cannabis at this stage of life may “perpetuate” these symptoms, triggering a psychotic episode that may result in a clinically well-established psychotic condition.
To summarise, we can ascertain that this study offers new evidence against cannabis use in adolescence. The results of the study suggest that it is advisable to use cannabis in moderation, rather than impulsively, especially if individuals have already experienced previous psychotic episodes.
Author: Javier Pedraza