> but the rate of overdoses from prescription opiates is, as i understand it, very low, because the dosage is predictable

Over time, addicts do not get the same high from the same dose of opiates. They require higher dosages, until such a point where they have to turn to illicit narcotics (which are also cheaper). Prescription drug abusers are 40x more likely of becoming heroin abusers.

This is all in the article I linked above (click on the subheadings).

if they could buy whatever dosage they needed over the counter, instead of trying to wheedle a prescription out of doctors who are under threat of criminal prosecution, they wouldn't have much of an incentive to switch to different opiates, or to rely on dangerously unstable supply chains

this would increase the risk of suicide, but it would have to rise a lot to exceed the current usa rate of accidental overdose deaths