Smoking a single joint of cannabis raises the risk of schizophrenia by more than 40%, reported the Daily Mail . Cannabis “could be to blame for one in seven cases of schizophrenia and other life-shattering mental illness,” it said.
The Times suggested that heavy users of cannabis “are more than twice as likely to suffer mental illness”. The stories were based on a study of the pooled results of several multinational studies that show a consistent link between cannabis use and psychotic illness. Reports say that this sheds new light on the need to issue warnings to people about the risks of smoking illicit drugs and also raises questions about the classification of the drug that is currently under debated.
The original research seems to have been well conducted and adds weight to the concern over the harmful effects of cannabis. However this study cannot prove that cannabis is a cause of psychosis or mental illness. However, because it is a review of many studies showing similar effects, it adds to the weight of evidence suggesting a link.
Where did the story come from?
The study was conducted by Theresa Moore, Stanley Zammit and colleagues from the departments of psychiatry and psychology in the universities of Cardiff, Bristol, Cambridge, and Imperial College, London. The research was funded by the Department of Health in the UK and was published in the peer-reviewed journal The Lancet .
What kind of scientific study was this?
This research is a systematic review that has used meta-analysis. This is a system of mathematically combining the results of several, separate studies to indicate whether smoking cannabis has any relation to psychosis and a range of mental illnesses.
The researchers conducted a search of several study databases to find studies that had examined populations over time and had looked at cannabis use. They looked at outcomes of psychosis, a general term for schizophrenia, or mood disorders. Data on the individuals within the studies or the detailed purposes of each study where not reported.
Thirty-five studies from Europe and the US were included. The researchers combined the results of studies where the people recruited to the studies and the methods used by the researchers were similar.
The results of seven studies that examined the effect of cannabis upon psychosis were combined. These individual studies had made an attempt to adjust for other factors that may have had a contributory effect, for example, whether the individuals had previous history of psychotic symptoms.
What were the results of the study?
The combined results of the seven studies that had looked at psychotic outcomes found that the odds of developing psychosis when a person had ever used cannabis in their life were increased by 41%. The combined results of studies examining more frequent cannabis use, showed an even greater chance of developing psychosis. Results and methods in the studies that had examined links between cannabis use and depression, anxiety and suicidal thoughts were less consistent, and no conclusive results could be drawn from these. For example, these studies included people with depression at the start and therefore the researchers were unable to say conclusively whether cannabis causes depression or whether people with depression tend to use more cannabis.
What interpretations did the researchers draw from these results?
The researchers concluded that they had found as clear evidence as is possible to find by observational studies, of a consistent link between cannabis use and psychotic illness. Any link between cannabis use and affective disorders, such as depression, is not clear. They say that there is enough evidence “to warn young people that using cannabis could increase their risk of developing psychotic illness in later life”.
What does the NHS Knowledge Service make of this study?
This research was well conducted and, by combining the results of several studies, it seems to present reliable evidence of a link between cannabis use and psychotic illness. The fact that the risk rises with the dose adds further weight to the suggestion that cannabis is one factor that causes psychosis.
However, several things need to be considered when interpreting these findings, which the researchers themselves acknowledge:
- Results of observational studies such as these cannot show cannabis to be a cause of psychotic illness, they can only show a possible link.
- Even though the researchers found a 40% increase in the risk of psychosis with cannabis use, the lifetime risk of psychosis, with or without cannabis use is still small (the study suggests a risk of less than 3%).
- Although attempts have been made in many of these studies to adjust for factors that may have a contributory risk of psychosis, aside from cannabis use, there are still many that may have an effect upon the results, for example, other drug or alcohol use, family history, sex, age, personal relationships, criminal involvement and so on.
- The size of individual studies and the methods used to examine cannabis exposure are variable; therefore their power to examine effects may have been different.
This research may provide the strongest evidence it is possible to obtain on the harmful effects of cannabis, without carrying out experimental or randomised studies. These are not possible or ethical for the illicit drug cannabis.