Valle del Vera, BelénParrado González, AlbertoCarmona Márquez, JoséFernández Calderón, Fermín2026-01-142026-01-142025del Vera, B. V., Parrado-González, A., Carmona-Márquez, J., & Fernández-Calderón, F. (2025). Explaining Avoidance of Simultaneous Alcohol and Cannabis Use: An Extended Theory of Planned Behavior Model Including Non-deliberative Processes. International Journal of Mental Health and Addiction. https://doi.org/10.1007/s11469-025-01567-21557-18741557-1882 (electrónico)https://hdl.handle.net/10272/27675Simultaneous alcohol and cannabis use (SAC) poses greater health risks than using each substance separately or concurrently. 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—non-deliberative processes—improved prediction; and (3) model invariance across sex and college status. A community sample of 378 young adults (60.3% men, Mage = 21.02, SD = 2.12) completed baseline and 3-month follow-up questionnaires. 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. The model’s predictive capacity was invariant across gender and educational status. While the TPB provides a strong framework for understanding health-promoting behaviors, our results highlight the importance of considering both automatic and reasoned processes.engAttribution 4.0 Internationalhttp://creativecommons.org/licenses/by/4.0/Simultaneous alcohol and cannabis (SAC) useHealth-promoting behaviorsTheory of Planned BehaviorNon-deliberative processesYoung adultsjournal article10.1007/s11469-025-01567-2open access6114 Psicología Social3212 Salud Publica