Analysis was made of the common sense explanations of 60 Mexican teenage illicit drug users in rehabilitation to determine their drug use debut. The explanatory model was separated into three blocks, two of which contained common sense aspects: interaction between subject's plane and the collectivity; and relationship between subject's interior (subject-family) and exterior (environment or group-pressure). Based on these data, we propose that drug use debut may be linked to subjects' emotional vulnerability which originates in the family image and renders them more susceptible to the influence of others. This kind of reasoning points to a more passive than active, and more social than personal dynamic in drug use debut, important factors for developing preventative measures. (Contains 4 tables.)