Dr. William Halsted, a great surgeon and one of the founders of Johns Hopkins was a morphine addict. Surprise surprise, they aren't all the domestic version of Viet Cong, despite what the Man tells us. Overdose is a probably largely due to people not knowing the purity of their H, the presence of adulterants which act in conjunction with the H, and addicts misjudging their tolerance. Using non-IV routes probably reduces the chances of ODing. Since abstinence is often forced, I can only imagine that most ODs could be avoided entirely by proper measures.
Tolerance is a funny thing.
Addicts have been known to die from their second shot of the day after dividing their daily amount into three piles. It would therefore seem that their tolerance had been reduced since the first shot. Someone conjectured that tolerance was partially a matter of place-conditioning and that addicts who shoot in a particular gallery get conditioned so that their body begins to gear up for a shot when they go their and that therefore they have higher tolerance there. This model works pretty well for cigarette companies. However, it is totally off the mark with respect to H sellers.