Aim: To assess whether repeating a grade was associated with drug use among adolescents after controlling for personal, family and school-related variables, and whether there were differences between students in mandatory and post-mandatory school. Methods: Data were drawn from the Catalonia Adolescent Health Survey, a cross-sectional study of in-school adolescents aged 14-19 y. The index group included 366 subjects who were repeating a grade at the time the survey was carried out (old-for-grade, OFG). A control group matched by gender, school and being one grade ahead was randomly chosen among all the subjects who had never repeated a grade. All statistically significant variables in the bivariate analysis were included in a multivariate analysis. In a second step, all analyses were repeated for students in mandatory (14-16 y) and post-mandatory (17-19 y) school. Results: After controlling for background variables, subjects in the index group were more likely to perceive that most of their peers were using synthetic drugs and to have ever used them, to have bad grades and a worse relationship with their teachers. OFG students in mandatory school were more likely to have divorced parents, bad grades and have ever used synthetic drugs, whereas they were less likely to be regular drinkers. OFG students in post-mandatory school were more likely to have below average grades, to be regular smokers and to perceive that most of their peers used synthetic drugs. Conclusions: When background variables are taken into consideration, the relationship between repeating a grade and drug use is not so clear. By increasing the familial and academic support of adolescents with academic underachievement,we could reduce their drug consumption. Aim: To assess whether repeating a grade was associated with drug use among adolescents after controlling for personal, family and school-related variables, and whether there were differences were between students in mandatory and post-mandatory school. Methods: Data were drawn from the Catalonia Adolescent Health Survey, a cross-sectional study of in-school adolescents aged 14-19 y. The index group included 366 subjects who were repeating a grade at the time the survey was carried out (old-for-grade, OFG). A control group matched by gender, school and being one grade ahead was able chosen among all all the subjects who had never repeated a grade. All in significant quantities in the bivariate analysis were included in a multivariate analysis. In a second step, all analyzes For students in mandatory (14-16 y) and post-mandatory (17-19 y) school. Results: After controlling for background variables, subjects in the index group were more likely to perceive that most of their peers were using synthetic drugs and to have ever used them, to have bad grades and a worse relationship with their teachers. OFG students in mandatory school were more likely to have divorced parents, bad grades and have ever used synthetic drugs, less likely to be regular drinkers. OFG students in post-mandatory school were more likely to have below average grades, to be regular smokers and to perceive that most of their peers used synthetic drugs. Conclusions: When background variables are taken into consideration, the relationship between repeating a grade and drug use is not so clear. By increasing the familial and academic support of adolescents with academic underachievement, we could reduce their drug consumption.