On-campus, drug-free policies are strictly enforced. Off campus, you could get away with it. There are options for people looking to drink or smoke pot, but no options for anyone looking into heavier options. Yes, pot is legal in Colorado, but only for those over the age of 21. Basically, if you look for alcohol or pot use, you can find it, but if you don't, you can live happily without ever running into it. There are some idiots that overuse and get out of control, but I've never personally seen them, I've only heard about them, and they're a very small majority. There's no peer pressure whatsoever. If you don't want to drink or smoke, no one is going to suggest you do. There's plenty to do without partaking, and you'll fit in perfectly fine with 99% of the population.