Cocaine and Ecstasy Cause DNA Mutation:

..."Cocaine and ecstasy have proved to be more dangerous than we had imagined,'' said Giorgio Bronzetti, chief scientist at the National Center for Research's (CNR) biotechnology department. "These drugs, on top of their toxicological effects, attack DNA provoking mutations and altering the hereditary material. This is very worrying for the effects it could have on future generations,'' he said...

I dated someone in college who tried to convince me a number of times to do Ecstasy with him. I researched the drug and found that I was too uncomfortable with possible side effects to really consider it, so each time I declined. In retrospect, I am glad I made that choice.