Please use this link to access this publication. Abstract Aims To examine changes in the evidence on the adverse health effects of cannabis since 1993. Methods A comparison of the evidence in 1993 with the evidence and interpretation of the same health outcomes in 2013. Results Research in the past 20 years has shown that driving while cannabis-impaired approximately doubles car crash risk and that around one in 10 regular cannabis users develop dependence. Regular cannabis use in adolescence approximately doubles the risks of early school-leaving and of cognitive impairment and psychoses in adulthood. Regular cannabis use in adolescence is also associated strongly with...