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 2,408 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.