Medical marijuana laws and adolescent marijuana use in the USA from 1991 to 2014: results from annual, repeated cross-sectional surveys

Abstract

Background: Adolescent use of marijuana is associated with adverse later effects, so the identification of factors underlying adolescent use is of substantial public health importance. The relationship between US state laws that permit marijuana for medical purposes and adolescent marijuana use has been controversial. Such laws could convey a message about marijuana acceptability that increases its use soon after passage, even if implementation is delayed or the law narrowly restricts its use. We used 24 years of national data from the USA to examine the relationship between state medical marijuana laws and adolescent use of marijuana.

Methods: Using a multistage, random-sampling design with replacement, the Monitoring the Future study conducts annual national surveys of 8th, 10th, and 12th-grade students (modal ages 13-14, 15-16, and 17-18 years, respectively), in around 400 schools per year. Students complete self-administered questionnaires that include questions on marijuana use. We analysed data from 1 098 270 adolescents surveyed between 1991 and 2014. The primary outcome of this analysis was any marijuana use in the previous 30 days. We used multilevel regression modelling with adolescents nested within states to examine two questions. The first was whether marijuana use was higher overall in states that ever passed a medical marijuana law up to 2014. The second was whether the risk of marijuana use changed after passage of medical marijuana laws. Control covariates included individual, school, and state-level characteristics.

Findings: Marijuana use was more prevalent in states that passed a medical marijuana law any time up to 2014 than in other states (adjusted prevalence 15·87% vs 13·27%; adjusted odds ratio [OR] 1·27, 95% CI 1·07-1·51; p=0·0057). However, the risk of marijuana use in states before passing medical marijuana laws did not differ significantly from the risk after medical marijuana laws were passed (adjusted prevalence 16·25% vs 15·45%; adjusted OR 0·92, 95% CI 0·82-1·04; p=0·185). Results were generally robust across sensitivity analyses, including redefining marijuana use as any use in the previous year or frequency of use, and reanalysing medical marijuana laws for delayed effects or for variation in provisions for dispensaries.

Interpretation: Our findings, consistent with previous evidence, suggest that passage of state medical marijuana laws does not increase adolescent use of marijuana. However, overall, adolescent use is higher in states that ever passed such a law than in other states. State-level risk factors other than medical marijuana laws could contribute to both marijuana use and the passage of medical marijuana laws, and such factors warrant investigation.

Funding: US National Institute on Drug Abuse, Columbia University Mailman School of Public Health, New York State Psychiatric Institute.

Figure 1

Medical Marijuana Laws and Years of Passage in the 48 contiguous U.S. States

Figure 2

Adjusted prevalence^a of last-month adolescent marijuana use by year (1991–2014), school grade (8, 10, 12^th), and U.S. state MML status ^aAdjusted prevalence estimates are derived from the multilevel model described in the methods section, fit to all 24 years of MTF data (1.13 million records) in the 48 contiguous U.S. states, with individual, school, and state level covariates fixed at the grade-specific overall U.S. distribution each year. Note that the 21 MML states passed their laws in varying years; thus, the yearly prevalence estimates for MML states are aggregated regardless of whether the state had passed a law yet.

Figure 3

Plotted log odds ratios for adolescent marijuana use pre vs. post passage of MML^{a a}Post-passage includes the year in which the law was passed. Values > 0 indicate increased log odds ratios of past month marijuana use post-MML compared to pre, values < 0 indicate a decrease. Error bars represent 95% confidence intervals for each state-specific estimate. State estimates for each grade are not shown when a state did not have pre or post MTF data available within that grade (for 8^th grade, NV does not have pre MML data and VT does not have any 8^th grade data. For 10^th grade, MT does not have pre MML data and RI does not have any 10^th grade data. For 12^th grade, DE, MT and NV do not have pre MML data, and NH does not have post MML data).