Most students at Carnegie Mellon stick to alcohol as their substance of choice, although the number of people who smoke weed is probably a close second. That's not to say that all students drink or smoke weed. It is very easy to spend four years here and not be tempted by others, as students genearlly don't heckle others for what they do or don't participate in. Drugs beyond alcohol and marijuana are generally uncommon, but some students feel they just can't handle all of the academic demands without using Adderall or Ritalin. Therefore, there is a moderately sized market for Adderall, Ritalin, and other drugs that focus on attention, especially around midterms and finals.
However, students who do partake in drug use other than alcohol have to be very hush-hush at CMU. Unlike getting caught with alcohol, where campus police might issue a citation but usually make you pour it out, campus police are told to call the Narcotics Squad if any significant quantities of drugs are found. As you might guess, the stakes are higher to participate in drug use, which is why a majority of students doesn't partake in it, and those that do are pretty secretive. Overall, it's the same as any school—students are told how "seriously" the school takes drug and alcohol use, but unless you're running a drug ring, you'll usually just get saddled with a citation if you get caught.