A difference-in-difference approach to examining the impact of cannabis legalization on disparities in the use of cigarettes and cannabis in the United States, 2004-17.
To estimate the impact of recreational and medical cannabis laws (RCL, MCL) on the use of cannabis and cigarettes in the United States. A difference-in-difference approach was applied to data from the 2004-17 National Survey on Drug Use and Health (NSDUH). United States. Nationally representative cross-sectional survey of Americans aged 12 years and older (combined analytical sample for 2004-17, n 783 663). Data on past-month use of (1) cigarettes and (2) cannabis were used to classify respondents into four groups cigarette and cannabis co-use, cigarette-only use, cannabis-only use or no cigarette or cannabis use. State of residence was measured by self-report. MCLRCL status came from state government websites. Difference-in-difference analyses suggest that MCL was associated with an increase in cigarette-cannabis co-use overall adjusted odds ratio (aOR) 1.09 95% confidence interval (CI) 1.02-1.16, with the greatest increases among those aged 50 years and above (aOR 1.60 CI 1.39-1.84), married (aOR 1.19 CI 1.07-1.31), non-Hispanic (NH) black (aOR 1.14 CI 1.02-1.07) and with a college degree or above (aOR 1.15 CI 1.06-1.24). MCL was associated with increases in cigarette-only use among those aged 50 years and above (aOR 1.07 CI 1.01-1.14) and NH black (aOR 1.16 CI 1.06-1.27) and increases in cannabis-only use among those aged 50 years and above (aOR 1.24 CI 1.07-1.44) and widoweddivorcedseparated (aOR 1.18 CI 1.01-1.37). RCL was associated with an increase in cannabis-only use overall (aOR 1.21 95% CI 1.09-1.34), a decline in cigarette-only use overall (aOR 0.89 95% CI 0.81-0.97) and increases in co-use among those who were married (aOR 1.24 CI 1.02-1.50) and aged 50 years and above (aOR 1.37 CI 1.03-1.84). Recreational and medical cannabis legalization have had a varying impact on the use, and co-use, of cannabis and cigarettes in the United States.