PROJECT SUMMARY The rates of cannabis co-use with alcohol and tobacco among young adults has continued to rise in recent years. Considerable research has shown that co-use places individuals at greater risk for driving under the influence, poorer mental-health and cognition, and co-morbid substance use disorders when compared to single-substance use. Despite the increased prevalence and risk associated with the co-use of cannabis with other substances, the mechanisms underlying these associations remains unclear. It has been hypothesized that individuals engage in co-use to achieve greater subjective responses beyond that perceived with the use of each substance alone. Given prior research has found inconsistent and null findings for outcomes related to patterns of co-use, this suggests changes may be complex and require more careful examination of within- person processes. Previous studies have also shown that subjective responses to cannabis may underlie or coincide the development of cannabis use disorder (CUD); however, it is unclear from the existing literature whether subjective drug effects of cannabis resulting from co-use with other substances, such as alcohol and tobacco, increases the risk for hazardous use over time. The proposed study will utilize advanced statistical methodology (e.g., multilevel modeling, longitudinal analyses) to examine ecological momentary assessment and longitudinal data in young adult recreational cannabis users from a NIDA R01 project. Aim 1 of the proposal will examine how cannabis co-use occasions with alcohol or tobacco (drug, ordering, timing) affects subjective drug abuse liability effects of cannabis; and Aim 2 will examine whether these subjective responses to co-use predict hazardous cannabis use one year later. Identifying underlying mechanisms of risk associated with cannabis co-use will help to develop better tailored intervention approaches to decrease the public health burden of CUDs among young adults.