From the documentary Heroin: Cape Cod, MA a potentially damning fact: 80% of heroin users started with painkillers. In the case of the 20-somethings in the documentary, most were crushing pills from their parents cabinets or (more usually) by abusing pills they received from doctors after accidents. When the pill addiction they developed became unmanageable they began use of heroin.
From another article:
Respondents who began using heroin in the 1960s were predominantly young men in their teens living in urban areas, whose first opioid use was heroin (80%). Recent users are more likely to be white, older (average age almost 23 years) men and women living in suburban or rural areas. Three out of four were first introduced to opioids through prescription painkillers. (Source)
Recent attempts to make Oxycontin more tamper-resistant have caused some addicts to move to heroin, which is cheaper.
“A few years ago when we did interviews with people in treatment, many would tell us that although they were addicts, at least they weren’t using heroin,” he said. “But now, many tell us that a prescription opioid might run $20 to $30 per tablet while heroin might only cost about $10.” (Source)
See Opioids, Alcohol, Suicide to understand the scope of the problem.