Procedures and measures
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MAY
The trial was approved by the UK Health Research Authority (13/EE/0303) and the UK Medicines and Healthcare Regulatory Agency (20,363/0325/001–0001) and was prospectively registered with ClinicalTrials.gov (NCT02044809) and the EU Clinical Trials Register (2013–000,361-36). The trial was a single-centre, randomised, double-blind, placebo-controlled and parallel-group study conducted at the Clinical Psychopharmacology Unit, UCL, in central London from May 2014–June 2017. Due to a lack of funding, a subsequent phase 2b stage trial that had been planned was not initiated, and the trial ended in May 2018. The trial protocol can be found at After an initial telephone screening, participants attended an in-person screening visit to determine their eligibility prior to randomisation. The trial statistician (GB) generated the randomisation sequence using block randomisation, with a block size equivalent to the number of treatment groups in the randomisation code. The randomisation code was held by the emergency unblinding service (Sealed Envelope, London, UK) and the drug manufacturer for labelling before shipping to the trial site. Researchers and participants remained masked for the duration of the trial. Only masked investigators enrolled participants, assigned participants to interventions, did assessments and entered data. Unmasking occurred after the database had been locked by the trial statistician.Synthetic, laboratory-synthesised CBD was obtained from STI Pharmaceuticals (Brentwood, UK) and manufactured by Nova Laboratories (Leicester, UK). The first treatment stage of the trial involved twice daily at-home ingestion of two gelatine capsules containing microcrystalline cellulose filler and CBD in total doses of 200 mg, 400 mg, 800 mg or 0 mg (placebo) for 4 weeks. Capsules were identical in size and participants were instructed to take each of the two doses 12 h apart. Text reminders were sent to participants at these pre-arranged times to improve compliance. Instructions were not given for taking the doses with/without food. Participants’ adherence to the treatment schedule was monitored via the return of dosette boxes and self-report of use using diary cards.The trial was conducted to determine the most effective dose of CBD in reducing cannabis use and consisted of two stages. In the first stage, Participants attended site visits once weekly during treatment. All participants received six 30-min sessions of motivational interviewing (Miller & Rollnick 