ENGLISH:
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.

FRANÇAIS :
Aux Etats-Unis, la consommation de cannabis des adolescents n'a pas augmenté dans les Etats où son usage médical est devenu légal. Cette nouvelle analyse de longue haleine menée sur 24 ans tend à démontrer que la décriminalisation du cannabis n'a pas modifié les comportements.
Le 15 juin 2015, des chercheurs de Columbia University à New York ont indiqué que la consommation de cannabis des adolescents n'a pas augmenté dans les Etats où son usage médical est devenu légal. Cette nouvelle analyse représente l'effort actuel le plus exhaustif à une question souvent posée : la décriminalisation du cannabis entraine-t-elle une consommation plus répandue chez les adolescents ? L'étude a trouvé que les Etats qui ont légalisé l'usage médical du cannabis présentaient des taux supérieurs de consommation adolescente avant la promulgation de la loi, par rapport aux Etats où le cannabis reste illégal, et que les modifications de la loi n'ont pas modifié ces taux.
Le rapport a été publié par le Lancet Psychiatry. Il couvrait une période de 24 ans jusqu'en 2014 et était basé sur des sondages réalisés auprès de plus d'un million d'adolescents de 48 Etats. La préoccupation première relative au débat sur le cannabis médical était de savoir si la diminution des restrictions envoyait un message contradictoire aux adolescents, et rendait le cannabis plus attractif et disponible. Grâce à plusieurs études, on sait que les adolescents qui développent une habitude quotidienne de forte consommation augmentent le risque de développer des difficultés d'ordre cognitif. [Résumé IDPC, 29/07/2015]