Is it inevitable that legalization of recreational cannabis will result in more dependence and psychosis? In theory, it is possible to legalize cannabis in ways that do not increase potency and prevalence of use but, so far, experience with commercialization in North America is not encouraging. Governments that decide to legalize cannabis should use some of the tax revenue to monitor cannabis price, consumption, and potency levels and to carefully evaluate the long term repercussions for mental health in different US states and Canadian provinces. Such monitoring would enable policies to be developed to minimize harm. In the absence of such an approach, it seems likely that the current commercialization of recreational cannabis in North America will be followed in a few years by a rise in the incidence of new cases of psychosis and in the prevalence of people with more chronic psychoses. (To find out why even this will fail, and only harms will increase see Then There Were Three – Marijuana Markets)