If you consider alcohol a drug, which by definition it is, then drugs are all over campus. Beyond drinking, however, the most popular drug is marijuana. Most students will at least end up trying marijuana by the time they leave UD if they haven't already. It is just around all the time. If you have no desire to be around marijuana, it isn't exceptionally hard to avoid it. You just hang out with those who are similarly minded.
Drugs that are more dangerous than marijuana are not so common. Ecstasy is the next most familiar, but it is rare to be offered ecstasy, or even to know someone is taking it. Most people will not offer you any of these hard drugs because they are expensive, and if they have an addiction, they don't want to share. While these drugs do show up from time to time, you should have no problems completely avoiding them should you so choose. You should have no worries about peer pressure; you definitely won't become a junky because someone else forces you!