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Introduction: For most drugs, age-specific incidence peaks during the midadolescent years and then declines. Alcohol is the exception with a viable explanatory theory of heterogeneous subgroups within the US population regarding attention to the law. A natural experiment of this hypothesis is underway as states legalize and regulate cannabis similarly to alcohol. We forecast the emergence of a bimodal cannabis incidence pattern as states set minimum cannabis purchase age at 21 years.
Methods: The population under study for these analyses are non-institutionalized civilian residents of the U.S., 12 years of age and older, as sampled and assessed for the National Surveys on Drug Use and Health (NSDUH), from 2010 to 2017. The sample is stratified by cannabis LMA policy- Colorado State and Washington State (CSWS) are the experimental group compared to the other 48 states as controls. We use two different methods of comparing age-specific cannabis incidence estimates by policy group. Results: The estimates using the panel study method for the 48 US states produced the traditional age-specific cannabis incidence pattern. In CSWS, however, the estimated cannabis incidence rises sharply at age 21, descending afterward. The incidence estimates at age 21 after implementation in CSWS quadrupled (from 5% to 21%). Discussion: These contrasting estimates do not yet qualify as strong evidence supporting the hypothesis that cannabis policy is shaping age-specific incidence in our hypothesized bimodal pattern. The lag time for seeing such policy effects might be as long as 5-10 years if cannabis follows the US experience with alcohol LMA. Keywords: Drug Policy; Marijuana; Cannabis; Age of First Use; Incidence; Drug Onset; Legal Minimum Age
Abbreviations: EMIRD: Extra-Medical Use of Internationally Regulated Drugs; LMA: Legal Minimum Age; NSDUH: National Surveys on Drug Use and Health; R-DAS: Restricted Data Access Portal; CI: Confidence Intervals; CSWS: Colorado State and Washington State; PDPD: Peak- Decline-Peak-Decline
