Comprehensive Cannabis Use Assessment

We obtained sociodemographic data using the Medical Research Council Sociodemographic Schedule, as described previously.¹⁴ (link) An updated version of the modified Cannabis Experience Questionnaire¹⁵ (link) (CEQ_EU-GEI) was used to gather detailed history of use of cannabis and other recreational drugs (appendix). To minimise recall bias, none of the recruitment materials for cases or controls mentioned cannabis or referred to its potential role as risk factor for psychotic disorder. Participants were asked if they had ever used cannabis in their lifetime; if the answer was yes, they were then asked to give details on their pattern of use. Questions on the type of cannabis used made no reference to its potency and allowed participants to report the colloquial name, in any language, of the cannabis they used.
We included six measures of cannabis use in the initial analyses, including lifetime cannabis use (ie, whether or not the individual had ever used cannabis), currently using cannabis, age at first use of cannabis,¹⁶ (link) lifetime frequency of use (ie, the frequency that characterised the individual's most consistent pattern of use), and money spent weekly on cannabis during their most consistent pattern of use. Using data published in the European Monitoring Centre for Drugs and Drug Addiction 2016 report¹⁷ that reported the concentration of Δ⁹-tetrahydrocannabinol (THC) in the types of cannabis available across Europe, supplemented by national data for each included country,18 , 19 , 20 , 21 (link), 22 (link), 23 , 24 (link), 25 , 26 (link) we created the final measure of cannabis potency (appendix).