Study intervention
Drugs used in the trial were contained in identical gelatin capsules containing microcrystalline cellulose filler and cannabidiol (0 mg, 50 mg, 100 mg, or 200 mg). Cannabidiol was obtained from STI Pharmaceuticals (Brentwood, UK) and manufactured by Nova Laboratories (Leicester, UK). During treatment, participants were instructed to take two capsules twice daily to achieve the daily doses of placebo, 200 mg CBD, 400 mg CBD or 800 mg CBD. Participants attended weekly site visits at baseline (day 0) and treatment days 7, 14, 21 and 28). Adherence was assessed by self-report at weekly site visits and by return of unconsumed capsules. Participants who did not show adequate adherence on any treatment week (≥30% capsules returned or ≥30% self-reported doses missed), or did not attend a site visit within two days of the scheduled appointment, or did take concomitant psychotropic medication during treatment, were not provided further CBD capsules for the duration of the trial but continued all other aspects of the protocol. Six 30-min individual sessions of motivational interviewing were provided to all participants by trained psychologists. These were provided during screening and weekly site visits. Motivational interviewing is a talking therapy that aims to help clients increase their readiness for change [