The co-occurrence of cannabis use and internalizing symptoms, such as depression and anxiety, during emerging adulthood (18–25 years) is well documented. However, while bidirectional relationships are often assumed, empirical evidence is mixed. This study investigates bidirectional longitudinal relationships between cannabis frequency and consequences and internalizing symptoms (depressive and anxiety) among high-risk emerging adults.

Data came from seven assessments collected over a 2-year period among 961 (54% female) high-risk emerging adults participating in two longitudinal cohorts (Ontario, Canada; Tennessee, USA). Assessments were at 4-month intervals spanning 2018–2020. Latent curve models with structured residuals were used to explore bidirectional between- and within-person relationships between cannabis-related variables and internalizing symptoms.

At baseline, higher levels of cannabis frequency and consequences were associated with higher internalizing symptoms. In between-person model components, cannabis-related and internalizing variables decreased across emerging adulthood. Significant within-person bidirectional relationships were observed, partially supporting both symptom-driven and substance-induced pathways, but the findings were specific to negative cannabis consequences, not frequency, and for depressive symptoms, not anxiety symptoms, for symptom-driven pathways. These bidirectional relationships were more pronounced among females and those surpassing clinical thresholds for internalizing symptoms at baseline.

This study found evidence of bidirectional relationships between cannabis consequences and internalizing symptoms across emerging adulthood, with the prevailing direction from cannabis-related negative consequences to increases in internalizing symptoms. These findings highlight the importance of cannabis intervention in emerging adults, both to reduce consequences and to prevent internalizing disorders, especially targeting females and those with clinically elevated internalizing symptoms.