They start with back pain due to a workplace injury, for which a doctor immediately prescribes them a powerful opiod (and perhaps a companion drug to deal with the resultant constipation).
Because it's "medicine", these people don't fully understand the risk of addiction nor do they recognize the signs of addiction and tolerance as they develop. Meanwhile, the doctor shirks their duty by not remaining in constant contact with the patient to manage any dependency should it occur.
These individuals eventually become addicted because the drugs lose efficacy over time (and in the case of Oxy, don't actually work for as long a period of time in the body as claimed, forcing people to up their own dosage). And thus the spiral of addiction begins.
Because of the lack of public awareness and the shame and stigma of addiction, people don't seek out help. Instead they try to manage the problem themselves. Ultimately, once addicted they turn to street drugs because they're a) cheaper, and b) more readily available.
Part of the problem is that, prior to the Oxy epidemic, the reality is doctors were probably under-managing pain, particularly for chronic pain sufferers.
Unfortunately, the reaction (again, in part due to drug company marketing) was to swing the other way and over-prescribe these drugs.
There was a Reddit comment on this subject to the effect of "I don't think of a muffin as breakfast, I think of it as my reward for getting up and going to work instead of hanging myself." And that sentiment is probably coming from a software developer in NYC or SF or whatever, so imagine how things look for everyone else.
There's one sure way to control an epidemic like this: education. That means educating the populace on just how addictive and dangerous these drugs are, educating law enforcement to treat addicts as people who are sick instead of simply stigmatizing criminality, and educating the medical community so that these drugs aren't handed out so casually, and in cases where they are appropriate, that patients are closely monitored for signs of addiction.
And the latter would benefit greatly from the US restricting drug company advertising and lobbying (this crisis was helped along significantly because of Purdue's marketing of Oxy in the 90s) so that medical decisions are made based on science and not marketing... but we know what the odds are of that happening.