A chemical that induces paralysis, pancuronium bromide can mask the signs of a painfully botched execution. In fact, its risks are so grave that 42 states ban its use for animal euthanasia. In his report released earlier this month, Ty Alper, associate director of UC Berkeley's Death Penalty Clinic, notes that nearly 98% of all lethal injections which used pancuronium brodmide to kill a human being took place in states where the law says it's inhumane to use the same or similar drugs to kill an animal.

Even the justices who signed on to the main opinion upholding lethal injection seem to understand the risks of the drugs currently used, and the potential for improvement. Justice Roberts, writing for the plurality, made clear that if alternative methods are proposed that are “feasible, readily implemented, and in fact substantially reduce” the risk of harm, a state would be violating the Constitution by not switching to them.

But the thing is, no hard evidence about such an alternative was presented in Kentucky's Baze v. Rees, so the justices could not choose a better procedure for Kentucky's death chamber. However, evidence that those alternatives exist abounds in other cases that originated in different states. For example, experts on both sides of the debate in Tennessee conceded as much, citing death by barbiturate as an effective and humane option. But the justices couldn't consider Tennessee's evidence because they were limited to Kentucky's. Commenting on today's opinion, Alper said, "In states where evidence of these procedures is introduced, unlike in the Baze case, it is reasonable to assume that the state's refusal to adopt those procedures will not be tolerated." In other words, lethal injection challenges will not cease with today's ruling and the debate about how to execute people, or whether to execute them at all, will certainly continue in states beyond Kentucky.

—Celia Perry