Polysubstance use is increasingly the norm among adolescents who use substances, yet research and clinical frameworks remain concentrated in single-substance models. This narrative conceptual editorial highlights key emerging literature on the conceptual inconsistencies and methodological challenges in polysubstance use research, particularly in the context of shifting cannabis legislation and adolescent vulnerability. A lack of definitional consensus, regarding the number of substances, timing of engagement, and intent of substances used, undermines the comparability of findings and the effectiveness of interventions. Adolescent polysubstance use is shaped by developmental, social, and structural factors, with cannabis and nicotine frequently overlooked despite their prevalence. This editorial addresses three linked gaps: definitional inconsistencies, insufficient attention to timing and intent of co-use, and the continued dominance of single-substance research frameworks. Therefore, we argue for clearer definitions, inclusive research designs, and interdisciplinary approaches.