We know how opioid addictions end: all too often, in death by overdose. About 16,000 Americans every year die of an overdose of opioid painkillers, such as oxycodone or fentanyl, according to the Centers for Disease Control and Prevention. We know less about how opioid addictions start. It's much harder to find and study people who start taking too many pills than it is to count dead bodies.