Physicians have long known that large doses of single drugs — certain sedatives or anesthetics — can take a life painlessly, and with far less distress than the three-drug cocktail causes if the injection is botched.

Since 2010, more death-penalty states — Oklahoma not among them — have moved to use single drugs for lethal injection. Even critics of the death penalty say most of those executions have gone more smoothly than ones involving multiple drugs.

Barbiturates, including sodium thiopental and pentobarbital, infused into the bloodstream can quickly make a person go deeply unconscious, stop breathing and die. Dr. Mark J. Heath, an anesthesiologist at Columbia University and an expert on lethal injection, said that high doses of pentobarbital were routinely used to euthanize animals, from pet rabbits to beached whales…

Even though Dr. Heath opposes lethal injection, he said, “I have not seen a single complaint, not an unhappy warden or family or anybody, from the single-drug barbiturate approach.”