For 2 decades, the US has been battling an unprecedented opioid epidemic. Fueled mainly by a precipitous rise in legal prescriptions for highly addictive painkillers, the number of opioid deaths in the US now has reached half a million. Experts warn that if we don't take more effective action, these potent opioids could kill millions more. Nora Volkow of the US National Institute on Drug Abuse joins us again for a deeper look at this issue.