There is little pressure for students to get involved in drugs or alcohol. SLU offers a variety of alcohol-alternative programming run by student groups, and there are tons of alcohol-free activities both on and off campus. Alcohol is much more popular than drugs, and the people who are into drugs generally stick together.
SLU is a "wet" campus if you're 21 or older, but alcohol is prohibited on campus if you are underage. If you're underage, don't try to party on campus. If you are caught for underage drinking, you'll have to attend a conduct hearing and take an alcohol safety class. The University is pretty strict about these rules, especially in the residence halls. Therefore, students often seek nightlife opportunities off campus. If you're 21 or older, the drinking scene is pretty big, and there are a ton of bars around the city. Still, no one will bother you if drinking isn't your scene, and there is still plenty to get involved in.