How do you deal with friends who drink?

I have two good friends who are smart, nice people but they've just made (what I think is) a stupid mistake. They've both started drinking...and they like it and don't regret getting drunk at all. They say their hangovers were bearable and that they want to do it again, and try drugs to just for that 'high' feeling. I've told them it's stupid, but they just don't regret it. They're planning to get drunk again soon, and I just don't think I can deal with their craziness! They're good kids, so good that I think that they're trying to 'rebel' or whatever...so how should I talk them out of it?