Objective: To study drug-related beliefs among adolescents, and specifically their propensity to distinguish "soft drugs" from "hard drugs"; to investigate factors associated with such a propensity as well as its relationship with cannabis use. Design & setting: A cross-sectional self-administered survey conducted among a random sample of 5,812 French high-school pupils. Method: A score was designed for assessing respondents' propensity to distinguish "soft" from "hard" drugs. The correlates were studied, and then used to model various levels of cannabis use. Separate analyses were performed for boys and girls. Results: The propensity to distinguish "soft drugs" from "hard drugs" was tightly associated with cannabis use. For both genders, exposure to preventive information during a course, or through posters or booklets had no impact on such a propensity, and indeed attending a conference on drugs reinforced this propensity among boys. For both genders, this propensity was also strongly associated with cannabis use, and exposure to anti-drug information through a specific lecture was positively associated with cannabis experimentation. Conclusion: The effectiveness of preventive information in French high schools is highly questionable, especially when it endorses an unbalanced "anti-drug" discourse that may even boomerang. As cannabis use and related beliefs are probably built together and reinforce each other, one should neither overestimate the impact of beliefs on behaviours, nor underestimate users' adherence to such beliefs. (Contains 3 tables and 2 figures.)