Drugs and relationships?

So I need some help, to put it into perspective I am a very good student going to a very college but I am still not socially awkward. When it comes to drugs and alcohol, however, I get really uncomfortable. Maybe it was my upbringing (blame both parents in medicine) or its just a mindset that I have developed on my own but I cannot overcome this huge discomfort. My girlfriend (who I love more than anything) does occasional "soft" drugs but plans later to do the crazier stuff. I don't know what to do to overcome my discomfort because I don't want to be the buzz kill for my girlfriend.....