Explaining Avoidance of Simultaneous Alcohol and Cannabis Use: An Extended Theory of Planned Behavior Model Including Non-Deliberative Processes Running title: Avoidance of simultaneous alcohol-cannabis use

Background: The simultaneous use of alcohol and cannabis (SAC), a common consumption pattern among young people, poses greater health risks than using each substance separately or concurrently. Understanding and promoting SAC avoidance is therefore critical. This two-wave prospective study examined 1) the Theory of Planned Behavior (TPB) capacity to explain the frequency of avoiding SAC use; 2) whether adding past behavior and habit strength -proxies for non-deliberative processes- improved prediction; and 3) model invariance across gender and college status. Method: A community sample of 378 young adults (60.3% men, Mage = 21.02, SD = 2.12) completed a questionnaire at baseline and three months later. Results: The TPB explained 55.9% of the variance in intention and 31.4% in behavior. Adding habit strength and past behavior raised explained variance to 73.6% and 49.9%. These non-deliberative factors were the strongest predictors, reducing the effect of traditional TPB components (only attitude remained significant). The model’s predictive capacity was invariant across gender and educational status. Conclusions: While the TPB provides a strong framework for understanding health-promoting behaviors, our results highlight the importance of considering both automatic and reasoned processes