Background: The study of marijuana use disorders is urgently needed because of increasing marijuana legalisation in multiple jurisdictions, the effect of marijuana use on future risk of psychiatric disorders, and deleterious effects of marijuana exposure. Thus, understanding trends of marijuana use and use disorders and examining factors that might drive these trends (eg, perceptions of harms from marijuana use) is essential.
Methods: We analysed data from US civilians aged 18 years or older who participated in annual, cross-sectional US National Surveys on Drug Use and Health from 2002 to 2014. The sample in each US state was designed to be approximately equally distributed between participants aged 12-17 years, 18-25 years, and 26 years or older. For each survey year, we estimated prevalence of marijuana use and use disorders, initiation of marijuana use, daily or near daily use, perception of great or no risk of harm from smoking marijuana, perception of state legalisation of medical marijuana use, and mean number of days of marijuana use in the previous year. Descriptive analyses, multivariable logistic regressions, and zero-truncated negative binomial regressions were applied.
Findings: 596 500 adults participated in the 2002-14 surveys. Marijuana use increased from 10·4% (95% CI 9·97-10·82) to 13·3% (12·84-13·70) in adults in the USA from 2002 to 2014 (β=0·0252, p<0·0001), and the prevalence of perceiving great risk of harm from smoking marijuana once or twice a week decreased from 50·4% (49·60-51·25) to 33·3% (32·64-33·96; β=-0·0625, p<0·0001). Changes in marijuana use and risk perception generally began in 2006-07. After adjusting for all covariates, changes in risk perceptions were associated with changes in prevalence of marijuana use, as seen in the lower prevalence of marijuana use each year during 2006-14 than in 2002 when perceiving risk of harm from smoking marijuana was included in models. However, marijuana use disorders in adults remained stable at about 1·5% between 2002 and 2014 (β=-0·0042, p=0·22).
Interpretation: Prevalence and frequency of marijuana use increased in adults in the USA starting in approximately 2007 and showing significantly higher results in multivariable models during 2011-14 (compared with 2002). The associations between increases in marijuana use and decreases in perceiving great risk of harm from smoking marijuana suggest the need for education regarding the risk of smoking marijuana and prevention messages.
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