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High-potency cannabis and the risk of psychosis

Abstract
Background

People who use cannabis have an increased risk of psychosis an effect attributed to the active ingredient δ9-tetrahydrocannabinol (Δ9-THC). There has recently been concern over an increase in the concentration of Δ9-THC in the cannabis available in many countries.

Aims

To investigate whether people with a first episode of psychosis were particularly likely to use high-potency cannabis.

Method

We collected information on cannabis use from 280 cases presenting with a first episode of psychosis to the South London & Maudsley National Health Service (NHS) Foundation Trust, and from 174 healthy controls recruited from the local population.

Results

There was no significant difference between cases and controls in whether they had ever taken cannabis, or age at first use. However, those in the cases group were more likely to be current daily users (OR = 6.4) and to have smoked cannabis for more than 5 years (OR = 2.1). Among those who used cannabis, 78% of the cases group used high-potency cannabis (sinsemilla, ‘skunk’) compared with 37% of the control group (OR 6.8).

Conclusions

The finding that people with a first episode of psychosis had smoked higher-potency cannabis, for longer and with greater frequency, than a healthy control group is consistent with the hypothesis that Δ9-THC is the active ingredient increasing risk of psychosis. This has important public health implications, given the increased availability and use of high-potency cannabis.

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Copyright
Corresponding author
Dr Marta Di Forti, Department of Psychiatry, Institute of Psychiatry, De Crespigny Park, London SE5 8AF, UK. Email: m.diforti@iop.kcl.ac.uk
Footnotes
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The study was funded by the Maudsley Charitable Fund and a UK National Institute of Health Research Biomedical Research Centre grant (BRC–SLAM).

Declaration of interest

None.

Footnotes
References
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  • ISSN: 0007-1250
  • EISSN: 1472-1465
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High-potency cannabis and the risk of psychosis

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eLetters

Reply to letter titled: Sample bias may obscure results

Marta M Di Forti
Clinical Lecturer
27 January 2010

We are grateful for the letter titled: sample bias may obscure results and its valuable comments. Nevertheless we are happy to challengeits conclusions.Among the socio-demographic variables we reported in table 1, it is correct to point out that unemployment rates are statistically significantly higher in the cases compared to controls (p<0.001). This difference has already been reported in previous epidemiological studies and there is no evidence that that this arises from a bias in the sample selection. However, it is rather a potential confounder. In our paper we did not discuss if or how employment status might have influenced our findings, because, together with other relevant variables, we controlled for it in the statistical analyses. Thus, the higher rate of unemployment in cases than controls might partially accounted for the drop of the crude OR=8.1 (95% CI 4.6–13.5) to the adjusted one OR=6.8 (95%CI 2.6–25.4), which occurred when we controlled for confounders including unemployment. However, the OR still remains strikingly high and statistically significant (p<0.05), indicating that our findings cannotbe explained by the effect of employment status or by any of the other social variables listed.Lastly we wish to comment on the suggestion that controls’ preference for low-potency cannabis might be consequent to their need to continue to be able to work. Wouldn’t this indicate that high potency cannabis is more likely to negatively impact on social functioning perhaps via its detrimental effect on mental health? Exactly what our findings suggest.

Marta Di Forti, Craig Morgan and Robin M Murray1. Di Forti M, Morgan C, Dazzan P, Pariante C, Mondelli V, Marques TR, et al. High-potency cannabis and the risk of psychosis. Br J Psychiatry 2009;195:488-91.
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Conflict of interest: None Declared

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Sample bias may obscure results

Euan M Lawson
GPwSI Substance Misuse
06 January 2010

Di Forti et al present their paper1 as further evidence of the link between high-potency cannabis and psychosis. Obviously, a major issue in case-control studies is the sampling and any difference between case and control groups needs to be carefully considered. The authors state that “there was no significant difference between the cases and control groups in age, gender, ethnicity, educational qualifications or employment statusat the time of assessment”. However, I would raise concerns about the employment status of the participants and I would respectfully highlight that this statement does not seem consistent with the information providedin the accompanying table of sample characteristics. This table states that 58.4% of cases and 43.2% controls were unemployed. The percentages inthis table have some inaccurate rounding but more worryingly, contrary to the authors’ report, there is a clear statistically significant difference(p=0.001 using a z-test for proportions).

This also seems to be a highly relevant and clinically significant difference that may have introduced considerable bias into this study and merited the attention of the 14 authors. In the discussion the authors state “the increased availability of skunk cannot alone explain why our control group members are less likely to prefer higher potency types than the cases group across time.” The requirement to hold down a job may be a highly significant reason why controls smoked cannabis of lesser potency less often than the unemployed. Moreover, individuals who are unemployed are highly likely to have poorer social and health status which further serves to obscure the true role of cannabis in this study.

1. Di Forti M, Morgan C, Dazzan P, Pariante C, Mondelli V, Marques TR, et al. High-potency cannabis and the risk of psychosis. Br J Psychiatry 2009;195:488-91.
... More

Conflict of interest: None Declared

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