Besides death, what can over dosing on pain medication lead to or caulse?

Near death. Liver and or kidney failure, stroke, heart attack, heart arrythmia, brain bleed, ruptured thoracic aorta, dissecting aneurysms, plung clots, leg clots, endocarditis, pericarditis, abscesses, etc. Doesn't sound good does it?
Stroke/encephalopath. If you od and stop breathing etc..Your brain will not get enough oxygen and may suffer anoxic encephalopathy (stroke is usually one area of brain, encephalopathy=entire brain) which maybe worse than death. Next bad thing is a heart attack. Maybe worse is you may not be looked at so nicely as a guy who abuses and od on narc... So, be good. Life is precious.. Good luck.