Introduction
Combustible tobacco use (mostly via cigarette smoking) in the United States (US) has seen a continual decline over the past several decades [The literature assessing the impact of co-use on treatment outcomes has been mixed. One systematic review found no clear effect of co-use on tobacco cessation [Another treatment-related concern among those who are co-using cannabis involves the potential for compensatory (increased) use of cannabis during tobacco treatment. Studies have either shown no differences in cannabis use during tobacco treatment or have demonstrated a reduction or concurrent decreases in cannabis use [With recent increases in the prevalence of tobacco and cannabis co-use among adults and the potential treatment implications of co-use, a better understanding of the relationship between cannabis use and its severity on tobacco cessation is needed. This trial protocol describes an ongoing prospective tobacco treatment trial in which adults who smoke cigarettes daily and are interested in quitting are enrolled (ages 18–40; N = 208), and participants who co-use cannabis are oversampled 2:1. This study has the following aims: (1) examine the impact of cannabis co-use on tobacco cessation among participants who are co-using compared to participants only using tobacco in a 12-week tobacco cessation trial; (2) using daily diaries and biochemical verification, assess changes in cannabis use (among those co-using cannabis) during tobacco treatment; and (3) assess for a dose-dependent impact of cannabis use on tobacco cessation outcomes.