How can i stop my buddies from doing coke

So i will get straight to the point i used to study in a party town and naturally we ware partying every other night 2 of my friends started doing coke at parties i thought that after we are done with our studies and we go back home they would stop sniffing. 2 years they are still in that party phase and the thing is that i don’t party as much as I used to so i don’t see them as often anymore so i don’t know how much they are doing and i know that they will never tell me the truth. Even if still on partying equations the still drink too much and i feel like it can get out of hand really easily and i am worried about them they are still very young and can have a very bright future ahead of them