Marijuana and tobacco co-use is common among young adults, and findings are mixed regarding the association between marijuana use and smoking cessation outcomes. This study examined the longitudinal relationships between marijuana use and smoking cessation outcomes among young adults (aged 18-25?years; N?=?500) enrolled in a 3-month smoking cessation intervention on Facebook. At baseline and 3, 6, and 12?months, participants reported their marijuana use and their smoking behaviors (seven-day point prevalence abstinence from smoking, cigarettes per day, quit attempts) and readiness to quit. Longitudinal analyses controlled for experimental condition and adjusted for baseline stage of change, baseline average cigarettes per day, sex, alcohol use, and age participants began smoking regularly. Use of marijuana by young adult smokers was associated with a lower likelihood of reduced smoking (OR?=?0.71, 95% CI [0.51, 0.98], p?=?.036) and a lower likelihood of abstaining from smoking (OR?=?0.56, 95% CI [0.35, 0.90], p?=?.017) in the past seven days, as assessed over 12?months of follow-up. Use of marijuana was not significantly associated with perceptions of or engagement in the smoking cessation intervention, stage of change for quitting smoking, or tobacco quit attempts (all p's?>?0.08). Study findings indicate that while marijuana use is unrelated to motivation to quit tobacco and engage in cessation interventions, marijuana use is associated with less success in reducing and abstaining from tobacco. Additional support and targeted tobacco cessation strategies to address challenges associated with marijuana co-use may be needed.
View details for PubMedID 30243420