The drug scene on campus is a fairly controversial topic. While some people will tell you that drugs are nearly non-existent, others find them readily and frequently available. It really depends on where you are on campus and who you know. However, the general consensus is that marijuana can be easily found and that cigarettes are commonly used. There isn't much (if any) pressure to be involved in such activities, but even if you don't participate, you can still see a fair amount of light drug use.
Heavier drug use can be found at off-campus parties, but it is not a big issue, if an issue at all, on campus. There is a problem however with students using light drugs outside of the dorms, which some non-using students find annoying. On that note, it isn't out of the ordinary to see someone hungover, but it is odd to see them uncontrollably high. While pot use may bother some students who can smell it outside of their dorm rooms, and cigarettes are used by many students in front of the buildings where other students walk, most other drugs won't influence the "substance-free" students at all. Their biggest inconvenience will usually involve alcohol, meaning that they might have to take care of a drunken friend on the weekend or a roommate after parties.