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Proportion of Patients in South London with First-episode Psychosis

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The rationale of the authors of this study was based on previous epidemiological research associating links for cannabis as a risk factor for psychotic illness such as Schizophrenia (SZ). The study by Di Forti et al. (2015) looked at this link between patients of first episode psychosis (FEP) and two types of cannabis, Hash (mild potency) and Skunk (high potency). Skunk has a higher potency of the chemical known to produce psychotic effects, ∆9 Tetrahydrocannabinol (THC) 15% compared to Hash 5%. Skunk also has minimal cannabidiol (CBD) less than 0.1% the other component in cannabis, known for its anti-psychotic effects (Freeman, 2015)

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A case study design; recruitment of participants with FEP (not identifiable with a medical cause) were from a clinic in the South London and Maudsley NHS Foundation trust, the control group were recruited from South London via newspapers, leaflets and the internet in an attempt to replicate the geographical population of the FEP group. Data collected was type of cannabis, Skunk, Hash or non-smokers, other substance use such as tobacco and alcohol in relation to frequency and lifetime use. The measured participants were 410 in the FEP group and 370 control group.

The results from this study shown on the demographics chart that the FEP group had more Males 271 compared to 209 controls, there were a high proportion of black Caribbean in the FEP group 136 to 73 control, but also higher proportion of 142 compared to 85 other black or ethnic minorities. Male ethnic minorities and especially black Caribbean males are at higher risk of psychotic illnesses, however adjustments for these possible confounders of gender and ethnic origin, as well as, cigarette consumption, units of alcohol, education and employment and lifetime history where made when using logistic regression statistics. 

Participants in the FEP group were more likely be daily users of cannabis, smoke Skunk and started smoking earlier at 15 years old compared to the control group. Participants who have never smoked cannabis are not at any increased risk of psychotic illness. However smoking cannabis at 15, moderately increased the risk, while smoking cannabis and Skunk increases a person’s risk of a psychosis by three times. Logistic regression test two, explored the frequency and type of cannabis smoked compared to participants who had never smoked cannabis. 

Hash users did not increase risk of psychosis dependant on frequency, but smokers of Skunk at least once a week were twice as likely to increase the risk; at weekends three times and daily were five times more likely to increase their risk of being diagnosed with a psychotic illness.  The use of a Population Attributable Fraction (PAF) calculation assessed the prevention of psychotic illness burden by cutting cannabis exposure was 19.3% for all cannabis, 24% Skunk users and 16% of daily smoked Skunk users. The research was an interesting read of the data and statistical figures which seem to support the notion that THC with its pro-psychotic effects can increase the risk of psychotic illness especially increasing the risk when heavy daily use of a high potency cannabis such as Skunk. 

The recruitment in one geographical area not only attempted to be representative of the study group population, but also heightened the plausibility that cannabis used would be of similar quality in the distributed area of South London. The study had a good population size with detailed environmental information was another strength. A cross sectional case study design are observational studies which do not support a causal link, therefore the study is not able to claim smoking high potency cannabis causes psychosis. As noted by the authors, other high-risk indicators for psychosis such as childhood sexual trauma, offspring of older fathers, poverty or ethnic minorities in urban areas etc., may encourage patients at the prodrome stage of psychosis to self-medicate with cannabis. 

No identifiable comparisons of psychotic risk in the study between smokers of Hash and smokers of Skunk was seen. Freeman (2015) questioned biological factors should be taken into account in further studies, that effects of pro-psychotic properties of THC high in quantity in Skunk may better explain these findings because Hash has less THC but higher percentage of anti-psychotic CBD.  The PAF is a causal calculation of prevalence in the wider population, the authors state if causation exists 24% of FEP may be attributed to high potency cannabis, however, acknowledge by the authors South London has high concentrations of high potency Skunk. 

This raises several questions one by Freeman for the enormity to public health, but also if this study population and concentration of Skunk in South London is representative of the wider population in the UK. Multi-measurements across the lifetime in longitudinal cohort studies and comparisons between low and high potency cannabis in relationship to psychosis would be a better design and improve validity and reliability. 

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