Saying he was now “paroled to my Father in heaven,” a convicted killer in Ohio on Tuesday became the first person in the United States to be executed with a one-drug intravenous lethal injection.

The new method, which involved a large dose of anesthetic, akin to how animals are euthanized, has been hailed by most experts as painless and an improvement over the three-drug cocktail used in all other states that employ lethal injection, but it is unlikely to settle the debate over the death penalty.

While praising the shift to a single drug, death penalty opponents argue that Ohio’s new method, and specifically its backup plan of using intramuscular injection if the authorities are unable to find a usable vein, has not been properly vetted by legal and medical experts. Since it had never been tried on humans before, they contend it is the equivalent of human experimentation.

But the United States Supreme Court refused to intervene on Tuesday morning, and the procedure went largely as planned.