The numbers suggest the majority started with a prescription. The number of people given these prescriptions was staggering around 4 times as many people got hooked (2.1 million) than are using Heroin (0.4 million). https://www.drugabuse.gov/about-nida/legislative-activities/...

"By 2002, death certificates listed opioid analgesic poisoning as a cause of death more commonly than heroin or cocaine."