The study, published in The Lancet Psychiatry journal showed no significant difference in adolescent marijuana use in 21 states with medical marijuana laws before or after implementation of these laws.

Since 1996, 23 US states and the District of Columbia have passed laws allowing the medical use of marijuana. State legalisation of medical marijuana has raised concerns about increased accessibility and acceptability of marijuana to teenagers.

In this study, Dr Deborah Hasin, Professor of Epidemiology at Columbia University Medical Center, New York, USA and colleagues examined the relationship between the legalisation of medical marijuana and adolescent marijuana use by analysing national 'Monitoring the Future' survey data* from over one million students in the 8th, 10th, and 12th grades (aged 13-18) between 1991 and 2014 -- a period when 21 contiguous states passed laws allowing marijuana use for medical purposes.