J.P. Abenstein, M.S.E.E., M.D., discussed the dangers of mixing fentanyl with heroin, a growing problem with illegal street drugs, with NPR Shots. "What happens is that people stop breathing on it," Abenstein explained."The more narcotic you take, the less your body has an urge to breathe. And it makes sense that a lot of people are overdosing on it because they aren't sure how much to take." Read more.