How do i get through to my friends?

Okay so me and my friends are sophmores in high school and recently they have started smoking weed. They think its harmless but now they are doing it almost everyday and with random people that they barely even know. What should I do, my sister told me I should tell their parents, but I feel like its not any of my business and I shouldn't get in it. I have a feeling that soon their going to get bored of weed so theyll move on to something stronger. Also they've even tried getting me to do it with them but I've made it very clear that I wont do that. Their personalities have changed alot and their starting to become total strangers to me.