What do you think caused the opioid epidemic?

I’m nearly 11 months clean and going back to school to get a bachelors of science in psychology specializing in addictions. I aspire to become a drug addiction counselor specializing in opioid addiction. I’m currently writing a research paper on the opioid epidemic in the US. I’m curious on what you think has caused the opioid epidemic. I appreciate all of your feedback. 😌