What advice are you looking for? She's not in the right. It sounds like she believes government and media propaganda and doesn't really know what she's talking about. That being said, you haven't said what your relationship with her is like otherwise. There are a number of things you could do depending on how much you care about her. It might not be worth your effort and you could end it, you could show her unbiased evidence to convince her otherwise or if you care enough about her you could stop smoking weed. To me, it sounds like the former two are the most likely options. You might want to show her Culture High, it's a documentary about cannabis.
The fact is you are perfectly within your right to smoke weed if you want to, and if she wants to hold such a view she's within her right to want you not to BUT she can't expect you to quit and you can't really expect her to want you to continue. As to how it's resolved, either one of you will have to give in and either you'll quit weed or she'll allow you to carry on smoking OR one of you will break it off with the other because of this difference in opinion. It sounds like she has her heart set on controlling you, on you stopping smoking rather than dumping you currently. If I were you I'd present these options to her and try to work this out but you have to be willing to accept that the result could be any of these options. You might have to break up with her, she might break up with you, you might stop smoking and obviously she might change her mind.