> 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).