Quantity of cigarettes smoked when co-used with alcohol and cannabis: Consideration of different definitions of co-use based on daily diary data

Introduction: Cigarettes are frequently co-used with alcohol and cannabis. However, definitions of co-use vary and the extent to which cigarette use changes on days with different patterns of co-use is unclear. We compared the number of cigarettes smoked on different days based on co-use patterns of cigarettes and alcohol or cannabis. Methods: This study analyzed 2408 smoking days collected in a 30-day smartphone-based daily diary study among 146 young adults (aged 18-26) with an oversample from sexual minority groups. Two separate multilevel models were estimated: one for cigarette and alcohol co-use and the other for cigarette and cannabis co-use. Both models examined day-level associations between the number of cigarettes smoked and 3 different types of days (smoking-only days, same-occasion co-use days, different-occasion co-use days), controlling for demographic characteristics. Results: More cigarettes were smoked on same-occasion co-use days compared to cigarette-smoking-only days for both alcohol (b=1.474, SE=0.136, t=10.8, p<.001) and cannabis (b=0.822, SE=0.209, t=3.9, p<.001). There were no significant differences in cigarettes smoked on days with co-use on the same day, but on different occasions, compared to days with smoking only. Conclusions: Compared to days with cigarette smoking only, more cigarettes are smoked on days when cigarettes are co-used with alcohol or cannabis on the same occasion, while the same is not true for days with co-use on different occasions. Conflating different definitions of co-use may impact findings on associations between co-use and smoking behavior.