Yale has an extremely tolerant policy on alcohol. Students are only disciplined for alcohol if they become involved with the New Haven (rather than the Yale) police or hospital. Instead of encouraging students to avoid the health center to avoid punishment, the University encourages students to seek medical help without fear of repercusion. These policies make drinking a very popular activity on campus since alcohol is widely available. If, however, alcohol leads to other problems like sexual harassment or vandalism, Yale will address the offences.
However, alcohol is not an imperative part of the Yale experienc. Students can have a great time (there are a lot of parties!) without ever touching alcohol. It's a personal choice for each student. There is so much to do on campus (dance parties, theater, music, comedy shows, etc.) that alcohol simply isn't necessary to have a fabulous night.
Like nearly every school, there is a drug scene amongst certain cliques or groups of students. However, it's easily avoidable and not an integral part of campus life. Alcohol is much more popular. If you don’t interact with people who use drugs, you might never even encounter drugs on campus. Students do not have to negotiate “peer pressure." Yalies value freedom of judgment and leave others' decisions up to them.