IU is a big school, where you can be as wild as you want to
be. There are plenty of police around to make sure that nothing gets out of hand, but there are still occasions where students will drink underage or smoke pot. Alcohol is by far the most prominent drug at IU, and that's the reason most students get in trouble. Drinking occurs even in the dorms, where the presence of liquor is strictly forbidden, along with some drug use, like marijuana. You're more likely to get caught trying something in an on-campus environment than you are elsewhere.
All in all, the drug scene on campus is easy to avoid. It seems to be the general consensus that there are drugs out there and that they are easily accessible-but perhaps no one is very interested. Those who wish to avoid any encounter with any drug may be lucky enough to do so. IU is a place where students care little about drug usage and almost never feel pressure to be involved.