Some half assed googling yielded this…

“there are significant differences between the way pets are euthanised and the way lethal injections are administered.

First of all, the typical three drug “cocktail” used in lethal injection is actually illegal to use on animals in most of the states that use it for executions because it is considered too inhumane. Almost all euthanasia by injection in the US is an overdose of barbiturate (sodium pentobarbital), which was originally used as an anesthetic agent. The animal being euthanised slips into unconsciousness very quickly, then the heart stops shortly thereafter. In humans, the 3 drug cocktail consists of some type of sedative followed by pancuronium (a curare derivative that causes paralysis but neither unconsciousness nor pain control) and finally a dose of potassium chloride to stop the heart.

Outlawing the use of barbiturates for lethal injection was a political decision made in the 1970s by politicians who did not want to be accused of treating human beings like animals. Disapproval of the use of curare derivatives for surgery or euthanasia in animals started in the late 1800s. But goodness, no one wants to be accused of treating a human being like an animal!”

Other reasons cited was that the human version is not produced on a mass or routine basis since demand is so low and the drugs can be old and may have lost potency…

I don’t know - just grab the horse version when it’s my time and send in the vet!