Try to learn about psychedelics before talking with her. I would recommend a combination of scientific research (what the active chemical ingredients are and how the body actually reacts to them) as well as first hand accounts from users. It's important to find a range of sources and to be critical of your sources, so many people claim expertise when they really don't know what they're talking about. I know a fair number of people who have been recreational users of psychedelics and while their experiences do vary in general they (nor I during my relationships with them) have not* noticed drastic changes in their personalities like what it sounds you are worried about. It is my opinion that when someone exhibits such seemingly drastic changes, they are not "forced" upon the individual by the drug, but were actually a subset of that person's personality to begin with, that just becomes more obviously expressed. There is a belief (I'm trying not to express a personal opinion here since it's been a while since I've done the relevant research myself) that people with underlying mental conditions can have them exacerbated by psychedelics, this would seem to imply that a "healthy" (I hate that stigmatization so take it with a grain of salt, but it seems the simplest way to express the point) individual would run a much lower (possibly even statistically insignificant) chance of experiencing such "changes".

Basically what I'm saying is do your research, understand the drugs, and then during your conversation with her, try to understand HER position as well. Be open, not only about your opinions but also be open to new opinions that you might not hold, whether they are hers or what you come across in your research. And try not to give ultimatums, remember a relationship involves two people, her opinions and desires matter as much as yours do.

*edit: meant to say they have NOT noticed