This study consists in the the design, implementation, and evaluation of the first web-based computer tailored intervention program aimed at the prevention of cannabis use in Spanish adolescents (ALERTA CANNABIS). A Cluster Randomized Controlled Trial is conducted to test the effectiveness of ALERTA CANNABIS in students aged 14 to 18 years across 34 high schools from Andalusia (southern Spain), which are randomized either to the experimental or the control condition (EC and CC).

Cannabis is the third most widespread drug and the most widely used illegal drug in Spanish and Andalusian adolescents aged 14 to 18 years, representing 8 out of 10 admissions to treatment for drug dependency in minors. It is necessary to continue investing in research applied to the prevention of cannabis consumption in order to develop scientifically validated strategies that can become preventive policies. The objective of the project is to contribute to the prevention of cannabis use in children, through the development, implementation and evaluation of the cost-effectiveness of a web-based-computer-tailoring and gamified learning intervention, having as reference the integrated theoretical model on adoption of health behaviors, the I-Change model. The starting hypothesis is that the application of this program, ALERTA CANNABIS, to children from 14 to 18 years old at the school context, will be cost-effective for reducing the prevalence of cannabis use.

A cluster randomized controlled trial will be carried out, with an intervention group, which will receive the ALERTA CANNABIS intervention and a waiting list control group. The following phases will be followed: design, implementation, cost-effectiveness evaluation (at 6 and 12 months), and dissemination of results.

ALERT CANNABIS will carry out an individualized advice, by means of pre-established messages, but generated based on the characteristics detected in a questionnaire (for example, the user's first name and taking into account the gender perspective, different types of reinforcement, etc.). In addition, it will use gamification techniques (videos, incentives, rankings) to encourage motivation and adherence to the intervention. The intervention will address motivational factors associated of cannabis use such as attitude, social influences, self-efficacy and intention.

The sample will consist of 1850 Andalusian students aged 14 to 18 enrolled in public high schools of the Andalusian provinces of Seville, Cordoba, Huelva, Cádiz, Granada and Malaga, 37 class groups by condition for the cluster design. Both, for the initial study and for the evaluation of cost-effectiveness, an online questionnaire applied in the school context will be used, with the variables resulting from the prevalence of cannabis use (in life, in the last 6 months, in last month), cannabis abuse using the cannabis abuse screening test (CAST) and health-related quality of life in terms of quality-adjusted life years (QALY). Confidentiality will be ensured in accordance with Royal Decree-Law 5/2018, of July 27, on the Protection of Personal Data. Informed consent will be requested.

If the cost-effectiveness of the program is demonstrated, it can be disseminated by becoming a public health policy.

Criteria: Inclusion Criteria:

• Students aged 14 to 18 years.
• Enrolled in Andalusian public high schools.
• Access to the Internet at the school and an equipped information and communication technology (ICT) room.

Exclusion Criteria:

• Language barriers.
• Previous participation in prevention programs for cannabis use.