Objectives: This study analyses changes in drug use in Mexican junior high and high-school students and identifies differences over two decades in the social factors (availability of drugs, perceptions of risk and social tolerance) associated with such use. Method: Data from two national surveys, conducted in 1991 and 2014, were analysed. Results: Data show that the proportion of students who had tried drugs doubled in 2014. In both surveys, substance use was significantly associated with a high perception of availability and use by friends and older siblings; in 2014, there was a decrease in the perception of risk for marijuana use and an increase in social tolerance toward illegal drugs. Conclusions: It can be inferred from this analysis that public policy to prevent drug use has not had the expected impact, at least not on the social factors considered here.