The opioid epidemic is directly caused by the OxyContin manufacturer's desire to sell a pain drug that "lasts twelve hours". These patient instructions almost couldn't be better designed to foment addiction. The big selling point was the "twelve hour" length of action. OxyContin was often not effective for the full twelve hours, causing a whiplash between under-treating pain and taking the medication at the prescribed time. This lead patients to be in pain for several hours every day, watching the clock as they waited for their next dose to alleviate said pain. If they take their medication early to combat this, they run out and have to deal with untreated pain until they get more. If they increase the dosage - as suggested by the manufacturer - then they now over-medicate at the start of the curve, build tolerance, and eventually become under-medicated again nearing the end of the twelve-hour period.

Psychologically, the risk of addiction comes from the brain getting trained that taking a substance causes a stark and rapid change in psychological state. If you have a consistent level of a medication, you tend to not become addicted to it. You can develop tolerance and physical dependence, but the addictive behavior doesn't get trained. To do that, you'll want a sharp and dramatic onset of effect associated with the drug-taking behavior, and negative reinforcement of the drug-taking behavior via the unpleasant effects of withdrawal.