Nowadays, cannabis has become the most prevalent using drug in most Western countries. With Canada become the second country following Uruguay to legalize recreational cannabis using, cannabis legalization is likely to become the trend in Western world. Though both Uruguay and Canada, as well as the states in the U.S. that legalized recreational cannabis using have the age restriction, a recent research indicated that cannabis using is spread among teenagers. With regard to the widespread use of cannabis and the legalization of recreational cannabis using in Canada and some states of the U.S., this study focuses on the effects of the legalization of cannabis using among teenagers to explore the reasons why teenagers use cannabis, why cannabis can be legalized, and what effects the legalization of cannabis may have on teenagers.