Girlfriend smokes, does pot and stuff?

Smoking weed and stuff is fine to me, but she does it a lot, and same with smoking. Whenever I see her smoke and do drugs I get really depressed because I don't want her to regret anything when she's older. I'm too scared to tell her to tone it down because I don't want to come off as telling her what to do, don't get me wrong, having one, two or even three cigarettes is okay with me, but not more.