It's been conventional wisdom for some time now that America's opioid epidemic began at the pharmacy. Now there are numbers that put any doubt to rest.

More than half of all people who succumbed to an overdose between 2001 to 2007 were chronic pain sufferers who filled an opioid prescription and sometimes even saw a doctor in the month before they died. Only 4 percent were ever diagnosed as having an abuse problem, said Dr. Mark Olfson, one of five researchers who conducted a massive study of the crisis and its causes for Columbia University Medical Center.

The findings of the new study, published Tuesday in the American Journal of Psychiatry, split the epidemic into two groups: those who were diagnosed with chronic pain and those who weren't. In the year before they died, about two-thirds of those studied were diagnosed with chronic pain and prescribed an opioid. (Many would also get a prescription for anti-anxiety drugs called benzodiazepines, which can make for a deadly combination.) The other third among those who died had no diagnosed chronic pain but became addicted to opioids in another way.