For the most part, the rumors are true—drugs are prevalent at Wesleyan. The two most popular substances are alcohol and marijuana, but you can find pretty much anything if you ask around. Many underage students drink (like at any university), and probably more than the average number smoke weed. Public Safety officers usually won't bust you unless they deem your situation to be dangerous or you show a blatant disregard for the rules (such as drinking/doing drugs openly in front of them). If someone is sick, P-Safe (Public Safety) will tend to their health first and foremost, calling an ambulance if necessary, and the first ambulance ride to the hospital is free.
However, if you choose not to participate in the drug scene, no one will look down on you; Wesleyan students are notoriously accepting of all and any life choices, including those related to drugs, alcohol, and smoking. In fact, sobriety is often respected and admired, which is something that differs from the state of affairs at other universities. There are substance-free halls available, and also a program house called "Well-Being House," where the residents sign a contract pledging not to bring any substances into the residence. At Wesleyan, drugs are regarded as one option among many weekend pastimes, and while there are a lot of them floating about, they by no means dominate the social scene.