The Joint Effort mobile application aims to support young adults into taking action on their cannabis use. Based on the Theory of Planned Behaviour, the content focuses on intention, attitude and perceived behavioral control. Various intervention methods and strategies are used to address these determinants (e.g., personalized feedback, persuasive communication, self-observation and activation of intention). The objectives includes: to allow the individual to become aware (or more aware) of their cannabis use, to support the individual's decision-making process of taking action on their cannabis use, to guide and support the establishment and sustainability of an action plan. An optional logbook-type feature (weekly journal of cannabis use) allows personalized monitoring and data collection throughout the course of the intervention.

The comparator is composed of a a brief normative feedback regarding last month frequency of cannabis use and basic reliable non personalized information on lower-risk cannabis use (official public websites).

The Joint Effort intervention is available in the form of an iPhone mobile application (running on iOS 13 and 14) in French language. It aims to support young adults in school who have used cannabis in the past month into taking action on their cannabis use.

Uptake is defined as the act of downloading and installing the mobile application (Szinay, Jones, Chadborn, Brown, & Naughton, 2020). The uptake rate will measure the proportion of participants randomized to the EG group that downloaded the mobile application (via the Apple iOS App Store) versus those who did not do it.

Subjective engagement (self-reported) will be measured by using the validated French version (Fontaine et al., 2020) of the User Engagement Scale - Short Form (UES-SF)(O'Brien et al., 2018). This 12-item instrument serves to measure four dimensions of engagement: 1) aesthetic appeal; 2) focused attention; 3) perceived usability; and 4) reward factor. Possible answers range from strongly disagree (+1) to strongly agree (+5). The possible total score ranges between 12 and 60. The score for each item will be interpreted and the scores for each dimensions will be compared to indicate which are rated more highly than other.

Objective engagement (ie. number of screens viewed by participants) will be assessed. This data will be collected automatically when users log into the application.

The intervention appreciation will be measured with the User Version of the Mobile Application Rating Scale (uMARS) (Stoyanov, Hides, Kavanagh, & Wilson, 2016). The uMARS is a 20-item measure that includes 4 objective quality subscales regarding engagement, functionality, aesthetics, and information quality and 1 subjective quality subscale. Each item is rated on a 5-point scale ranging from inadequate (+1) to excellent (+5). The possible total score ranges between 20 and 100; a higher scores indicates a higher appreciation.

The recruitment rate (ie. number of participants who signed the inform consent form) will be assessed.