Not sure what kind of master's program you are in, but you may run into liability issues with this. Marijuana is still a federally prohibited substance, and most schools aren't going to approve data collection of this nature by a master's student. I've seen proposals for research on this that is designed around anonymous self-report surveys, but even this can be risky and the anonymizing may compromise your data. Obviously there are experimental studies (very interesting work being done on psilocybin), but this requires serious institutional support and investment from whoever is approving your IRB. As a master's student, your best bet with this line of research would be to find a program where this research is already under way, and then your thesis could be a subset of that investigator's work. Good topic, just LOTS of hurdles around the logistics of this research that may be difficult to get through if you are wanting to "solo" this question as a master's student.