CANreduce 2.0 - comparing two differently optimized versions of a web-based self-help program to reduce cannabis use with each other and a waiting list

INTERVENTION: Participants are randomly allocated to one of three arms: 1. Intervention group: Adherence‐focused guidance enhanced web‐based self‐help program 2. Active control group: Unenhanced web‐based self‐help program 3. Waiting control group: Waiting list CANreduce is an automated web‐based self‐help tool based on classical CBT approaches for treating cannabis dependence. It will consist of a consumption diary, eight modules designed to reduce cannabis use based on the principles of motivational interviewing, self‐control practices, and methods of cognitive behavioural therapy ( strategies for goal achievement, Identifying risk situations, Dealing with cannabis craving, Dealing with relapses, Working on needs, Saying “no” to foster refusal skills, Dealing with burdens, Preserving achievements). Participants can study all modules at their own pace and order, though a specific order will be advised. The control condition is a waiting list. After 3 months their study phase is finished and they will be given the opportunity to start the self‐help program. Furthermore, there will be an active control group which works with the unenhanced version of the self‐help tool. The masking technique will be single‐blind, i.e. participants will not know if they work with the enhanced or unenhanced version (however, they will know if they have been assigned to the waiting list). Once participants have completed their baseline assessment, they will be randomized by a computer program in a 1:1:1 ratio to 1 of 3 “parallel” groups, and this assignment will be automatically registered in the background database. As participants will see if they have been assigned to the waiting list, there is a risk that some might try to circumvent their assignment by registering another account in hope to end up in a different group. In case a participant surmounts the administrative hurdle, he nevertheless will be assigned to the same group for a certain amount of time, based on his IP‐address. CONDITION: Problematic cannabis use ; Mental and Behavioural Disorders PRIMARY OUTCOME: Quantity and the number of days per week of cannabis use. Participants enter their cannabis consumption quantity and frequency into their consumption diary every week.; ; Timepoints of measures will be initial assessment, after 6 weeks and at 3 months follow‐up. SECONDARY OUTCOME: 1. Severity of cannabis dependence (SDS); 2. The use of alcohol, tobacco, and other non‐cannabis illicit drugs (FDA); 3. Changes in depression, anxiety and attention deficit symptoms (CES‐D, GAD‐7 and ASRS‐V1.1); 4. Post traumatic Stress Disorder (Short Screening Scale for DSM‐IV); 5. Client satisfaction (ZUF‐8); ; Severity of cannabis dependence and the use of alcohol, tobacco, and other non‐cannabis illicit drugs will be measured at initial assessment, after 6 weeks and at 3 months follow‐up.; ; Changes in depression, anxiety, attention deficit symptoms and the Short Screening Scale for DSM‐IV Post traumatic Stress Disorder will be measured at initial assessment and at 3 months follow‐up. ; ; Client satisfaction will be measured after 6 weeks. INCLUSION CRITERIA: 1. Cannabis use of at least once a week 30 days prior to study entry 2. At least 18 years old 3. At least weekly internet access and a valid email address