I have had to euthanize mice for experiments before. The animal care people here are very adamant that the animals experience the least amount of discomfort as possible, as they should, it's their job. One acceptable way to euthanize a mouse is an intraperitoneal injection of euthasol, which is a barbituate, a class of CNS inhibitors commonly used as anesthetic.
If this is the most humane way to euthanize that should mean that lethal injections, as practiced by countries that euthanize convicted criminals as part of capital punishment, should be the same. I guess, that is, assuming that the point of lethal injection is to be more humane than previous methods of euthanization. I was surprised to learn that lethal injection is not very simple.
Lethal injection in the United States consists of three sequentially administered drugs. These drugs are sodium thiopental, pancuronium bromide, and potassium chloride given intravenously in that order. This is a similar procedure to what is on wikipedia for physician assisted suicide.
Sodium thiopental, also used as 'truth serum', induces relaxation and release from social inhibitions. It is a short-acting barbituate that at the doses for lethal injection rapidly induces a coma. It inhibits CNS activity by activating GABAa receptors. Alcohol acts on these same receptors, however, barbituates are not used much recreationally anymore, I think, as they have been largely replaced by benzodiazepines. 'benzos' (apparently safer and more fun). Relevant to lethal injection is that sodium thiopental has no analgesic effects ie. not a painkiller.
Then comes the pancuronium bromide. This blocks acetylcholine at neuromuscular junctions, rendering the convicted paralyzed, including breathing muscles. Hopefully that thiopental coma is going well or it would be like drowning.
Then just to ensure death, potassium chloride. Such a simple salt of two ions that are very common in your body is deadly? Really it is because the large bolus IV injection at a high dose messes with the electrochemical gradient needed for proper cardiac muscle function. The heart is stopped.
I wonder how much this combo has to do with ensuring death, and making it less traumatic for the witnesses than it does with being humane. A good way to go might be an overdose of an opiate or something but seeing a convict get all high and say weird things might be too much.