Covariates in studies examining longitudinal relationships between substance use and mental health problems among youth: A meta-epidemiologic review

INTRODUCTION: This meta-epidemiological review examines covariate selection and reporting practices in observational studies analyzing longitudinal relationships between youth substance use and mental health problems (internalizing and externalizing). METHODS: Sixty-nine studies published in high-impact journals from 2018 to 2023 were included. Studies were included if they explored prospective relationships between substance use and mental health among youth (12-25 years) and used repeated measures designs. Data extraction focused on study characteristics, covariates and their selection methods, and reporting practices. RESULTS: There were 574 covariates included across studies; 33 were included as moderators and 18 were included as mediators. At the study level, the most common covariate domains included demographics (90 % of included studies had at least one demographic, mostly sex), substance-related variables (67 %; mostly alcohol or smoking), internalizing symptoms (39 %; mostly depression), family-related variables (29 %; mostly parental substance use or mental illness), and externalizing symptoms (19 %; mostly conduct). 93 % of studies had unique sets of lower-order covariates. Across all studies (n = 69), only 35 % provided details for how, and why, all covariates were selected with only 12 % reporting selecting covariates a priori, and none being pre-registered. Only 60 % mentioned confounding and only 13 % mentioned risk of confounding in their conclusions. CONCLUSIONS: The findings highlight the need for improved covariate selection and reporting practices. Establishing a core set of covariates and adhering to standardized reporting guidelines would enhance the comparability and reliability of research findings in this field. Researchers can use this review to identify and justify the inclusion and exclusion of commonly reported covariates when analyzing relationships between youth substance use and mental health problems.