Drugs exist on any college campus, and Pitt is about on par with other large public schools as far as prevalence goes. If you're in a position to see the drug scene at all, most of what you'll encounter will be alcohol and marijuana. Ecstasy, acid, and occasionally other "hard" drugs may appear, but use of these is even more subdued than marijuana. Although most of the common hard drugs can be found somewhere in the city of Pittsburgh, they've largely stayed outside of campus and away from Pitt students. All drug use around campus is discreet; many students have never personally encountered the drug scene at all, or they have only seen it from a distance at parties. You shouldn't need to worry about any pressure to use drugs, as long as you're careful about whom you make friends with. A good bit of the University's social scene includes, or revolves around, alcohol, and all but the most anti-social underclassmen are bound to encounter it at some point.
Even so, there still isn't a great deal of pressure to drink-again, this depends a lot on where you spend your time and how you choose your friends. It's important to remember that, for all the drunken parties you can find each weekend, there are also plenty of student group events and other things going on, both on and off campus, that don't involve alcohol at all. The decision to drink is left to you.