STUDY PROTOCOL Open Access CANABIC: CANnabis and Adolescents: effect of a Brief Intervention on their Consumption – study protocol for a randomized controlled trial

Background: Cannabis is the most consumed illegal substance in France. General practitioners (GPs) are the health professionals who are most consulted by adolescents. Brief intervention (BI) is a promising care initiative for the consumption of cannabis, and could be a tool for GPs in caring for adolescents who consume cannabis. The aim of the CANABIC study is to measure the impact of a BI carried out by a GP on the consumption of cannabis by adolescents of 15 to 25 years of age. Methods: A randomized clustered controlled trial, stratified over three areas (Auvergne, Languedoc-Roussillon, and Rhône- Alpes), comparing an intervention group, which carries out the BI in consultation, and a control group, which ensures routine medical care. The main assessment criterion is the consumption of cannabis by amount of joints per month, at 12 months. The amount necessary to highlight a significant difference between the two groups of 30 % of consumption at 12 months is 250 patients (50 GPs, 5 patients per GP; risk α = 5%; power = 90%; intra-cluster correlation coefficient ρ = 0.2; Hawthorne effect = 15%; lost to follow-up rates for GPs = 10 % and for patients = 20%). This plan is replicated for the three areas, and therefore a total of 750 patients are expected. The secondary criteria for judgment are the associated consumption of tobacco and alcohol, the perception of the consequences of consumption, and the driving of a vehicle following consumption