In this cross sectional and descriptive study, secondary school students trom the city of Santa Rosa were questioned about their beliefs about cannabis and their risk perception derived from it. The sample consisted in 83 male and 71 female 17 year-old teenagers. On the one hand, it was found that the highest risk perceptions were related to the legal issues that might arise due to cannabis consumption, and to its effects on neurons. On the other hand, the lowest risk perceptions were associated with the belief/ idea that smoking tobacco affects the lungs more than smoking cannabis, which might create dependence, and its use can cause mental disorders. Several significant differences were found as regards gender, since the female students noticed more risk than male students in that the consumption of cannabis can develop mental disorders, amotivational syndrome, lack of enthusiasm and less satisfaction with life. The teenager's risk perception about cannabis is variable.