Project Abstract Cigarillos are popular among youth and young adults and are commonly concurrently used with cannabis. Co- use of tobacco and cannabis is associated with increased frequency of and dependence on both tobacco and cannabis, and possible increased health risk compared to use of either substance alone. Qualitative research suggests that flavored cigarillos are preferred for cannabis/blunt use because the flavoring masks the smell and enhances the experience of creating and smoking blunts. A gap in the current literature exists for characterizing flavors in cigarillos and their impact on patterns of co-use of cigarillos and cannabis among young adults. It has yet to be determined whether the removal of flavors from cigarillos would impact co-use of cigarillos and cannabis, and whether that impact is related perceptions of appeal or risk. To accomplish this, we have proposed two integrated aims, building off of the parent grant (R01DA048529), to gather data to inform CTP regulation strategies on flavored tobacco, specifically for cigarillo products. First, through an analysis of cross-sectional data on young adult cigarillos users collected in the parent study, we will examine, through structural equation modeling, the relationship between use of flavored cigarillos and co-use of cigarillos and cannabis (including blunts), including motivation to quit each product. Within the same sample, we will recruit a sub-set of participants to conduct one-on-one qualitative semi-structured interviews. We will explore topics to expand findings from the parent study, including understanding components of flavor appeal, perceived harm and product substitution. We will also assess these factors in the context of co-use with cannabis to gain a deeper understanding of co- use patterns in this supplement. Through these two aims, this supplement will extend the objectives of the parent study by examining the influence of flavor on young adult co-use of cannabis and cigarillos. Resulting data from this study will inform policy development regarding potential effects of tobacco regulatory measures on co-use with cannabis.