In 2013 Beau Kilmer, co-director of the RAND Drug Policy Research Center in Santa Monica, California, led a team to develop a web-based survey that would ask people how often they had used cannabis in the past month and year. To help them gauge the amounts, the surveys included scaled pictures showing different quantities of weed. The survey, along with other data the team had collected, revealed a rift between perception and reality. Based on prior data, state officials had estimated use at about 85 tonnes per year; Kilmer’s research suggested that it was actually double that, about 175 tonnes1. The take-home message, says Kilmer, was “we’re going to have to start collecting more data”.
Despite claims that range from its being a treatment for seizures to a cause of schizophrenia, the evidence for marijuana’s effects on health and behaviour is limited and at times conflicting. Researchers struggle to answer even the most basic questions about cannabis use, its risks, its benefits and the effect that legalization will have.