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Decreasing smoking initiation remains a public health priority. In 2016, California, in the United States, enacted the Tobacco 21 law, which raised the minimum age for the purchase of tobacco products from age 18 to age 21. This paper evaluates whether the enactment and implementation of the Tobacco 21 law changed how young adults perceive the risk(s) of smoking. Data were drawn from a cohort of emerging adults (n = 575) in California who were non-daily smokers at enrollment and followed quarterly for 3 years. Data were collected during 2015–2019. Piecewise multilevel regression models were used to test for changes in smoking status and perceived risks of cigarettes after Tobacco 21 enforcement began. Findings indicated that the prevalence of current smoking and perceived risks of smoking both declined following Tobacco 21 implementation (ps < 0.001). Post-hoc analyses suggested that post-implementation changes in perceived risk occurred primarily among ongoing smokers. Findings suggest that Tobacco 21 and associated public health measures have been effective, but additional research is needed to disentangle the effects of specific components. Understanding the impact and efficacy of tobacco laws provides great social value to research and implement policies that create intervention(s) on reducing tobacco use initiation.