Objective: Research on substance use motives typically examines each substance separately. However, simultaneous alcohol and marijuana (SAM) use-that is, using alcohol and marijuana at the same time so that their effects overlap-is common among young adults. This study examines day-to-day fluctuations in motives for using alcohol and/ or marijuana among young adult substance users as predictors of alcohol, marijuana, and SAM use across days.
Method: Data were from a community sample of young adults who reported SAM use in the past month (analytic sample: N = 399, mean [SD] age = 21.63 [2.17]; 50.9% women). Participants reported alcohol, marijuana, and SAM use, and also motives "for alcohol and/or marijuana use" for 14 consecutive days.
Results: Multilevel models showed that elevated enhancement motives were associated with heavy episodic drinking, drinking more, and more hours high from marijuana. Elevated social motives were associated with heavy episodic drinking and drinking more, and also with fewer hours high. Elevated conformity motives were associated with drinking more. SAM use was more likely: on alcohol days and on marijuana days with elevated enhancement and conformity motives, on alcohol days with elevated coping motives, and on marijuana days with elevated social motives.
Conclusions: SAM use on a given day was primarily associated with enhancement and conformity motives. Social motives were more strongly linked to alcohol use, and to some extent coping motives were linked to marijuana use in this young adult sample. Further examination of situation-specific motives and contexts of use is needed to inform development of real-time interventions for SAM use and consequences.