There are a multitude of reasons to explain how this happened but to quickly sum up an excellent book:
- Purdue created the whole "sell-direct-to-doctor" phenomena that is now the norm in the US medical profession
- One bad study that showed opiates for pain relief are NOT addictive and this study kept being cited by sales people
- Mexican drug dealers from a very tiny area in Mexico importing black tar heroin
- A prevailing idea in the US that people should never be in pain and managing it through lifestyle changes is not acceptable; a quick fix is needed
- economic depression in the Midwest and Appalachia regions
But really, read the book. It's eye opening and well written.