Significance: Cigarette smokers are more likely to use cannabis than non-smokers. Couse may impede successful smoking cessation. This study examined whether changes in cannabis use frequency influenced tobacco smoking cessation outcomes.

Methods: A prospective cohort of representative samples of adult smokers (n=4,480) from Canada, the United States, England, and Australia were surveyed in 2016 and 2018. In each year, smokers were asked how often they typically used cannabis in the previous 12 months. Self-reported use in each year was compared to determine whether usage frequency increased, remained unchanged, or decreased between years. Smoking cessation outcomes, measured in 2018, were any attempt to quit in the previous year, having quit smoking, and having quit for at least 6 months. Weighted multivariable logistic regression estimated the odds of each cessation outcome as a function of changes in cannabis use controlling for sociodemographic and behavioral covariates.

Results: Eight percent of smokers who started using cannabis/increased their frequency of use had quit smoking tobacco by 2018 compared to 12% of smokers who did not use cannabis in either year (adjusted OR=0.50, 95% CI: 0.30-0.83). Significantly fewer smokers who started using/increased their frequency of use reported having been quit for at least 6 months by 2018 compared to smokers not using cannabis in either year (2% vs. 8%, respectively, adjusted OR=0.29; 95% CI: 0.14-0.60). Smokers who reduced their usage frequency/stopped using did not differ from non-users in their odds of quitting.

Conclusions: Smokers who increased their frequency of using cannabis were less