Capital punishment

Interesting question and answer from Cecil Adams at The Straight Dope. Also shows how barbaric the US looks to the civilized world.
Why isn’t heroin used for lethal injections?
October 30, 2015
Dear Cecil:
Hi mate. A question from no-capital-punishment Australia: I’ve noticed how difficult it’s become for you guys to get the necessary goop for lethal injections, with chemical companies refusing to supply it, etc. (when a chemical company is worried about its reputation, then yikes), leading to some states mixing their own cocktails, with horrific results. My question is: Why don’t they use heroin or some other opiate? Isn’t an overdose a reliable way of killing someone, shutting down breathing, consciousness, and pain? It’s also easy to get and make. If they’re not dead, just add a bit more and they soon will be.
— Andrew McDonald,H137,2ZPVSG,13SXM,1