Florida's lethal injection gurney is shown in an undated handout photo taken in the redesigned death chamber that will accommodate either the electric chair or the gurney at Florida State Prison. Lethal injections were supposed to be the humane way to execute inmates and avoid the excruciating pain caused by firing squads, electric chairs and the hangman's noose. But many still argue the chemical cocktail amounts to cruel and unusual punishment.

JACKSONVILLE, FLA. ? Lethal injection was supposed to be the humane, enlightened way to execute inmates and avoid the pain and the gruesome spectacle of firing squads, the electric chair and the noose.

But now it, too, is under legal attack as cruel and unusual, with the U.S. Supreme Court agreeing this week to hear arguments that lethal injection can cause excruciating pain.

Some supporters of the procedure say the notion that inmates suffer is unproven. And they argue that there is nothing wrong with lethal injection itself; instead, they say, the problem is inadequately trained executioners.

In fact, the man who developed the procedure 30 years ago said it is similar to the simple injections given every day in hospitals.

“What causes it to go wrong is that the protocols aren’t carried out properly,” said Dr. A. Jay Chapman, former Oklahoma medical examiner.

If an execution is about as simple as an ordinary injection, what, then, can go wrong?

In the three-drug process used by most of the 38 states that practice lethal injection, sodium pentothal is given first as an anesthetic and is supposed to leave the inmate unconscious and unable to feel pain. It is followed by pancuronium bromide, which paralyzes the inmate’s muscles, and then potassium chloride, which stops the heart.

Foes of capital punishment argue that if the inmate is not properly anesthetized, he could suffer extreme pain without being able to cry out.

That could happen in a number of ways: The executioner could inaccurately calculate the dosage needed for an inmate of a given body weight. Or the executioner could fail to administer the full amount, mix the drug improperly, or wait too long between giving the anesthesia and the lethal substance.

In Missouri, a doctor who participated in dozens of executions was quoted recently as saying he was dyslexic and occasionally altered the amounts of anesthetic given.

A botched execution in Florida last year illustrated another way a lethal injection could go awry: Angel Nieves Diaz needed a rare second dose of chemicals – and the execution took a half-hour, twice as long as normal – after the needles were mistakenly pushed clear through his veins and into the flesh of his arm. That left chemical burns in his arm that opponents say probably caused him extreme pain.

During the process, Diaz appeared to grimace. But he did not specifically say he was suffering. And a state panel was unable to determine if Diaz had been properly sedated or if he felt pain.

There is no direct proof that inmates have suffered while undergoing lethal injection. After all, they don’t live to tell about the experience.

But opponents of lethal injection often cite a 2005 study in the British medical journal The Lancet indicating that the anesthetic can wear off before an inmate dies. The study involved 49 U.S. executions. In 21 of the deaths, the study found, inmates were probably conscious when they received the final drug that stops the heart.

Chapman said that he has not seen definitive proof inmates suffer, and that, in any case, the pain would be small.

“Who’s to say exactly how much pain that an individual – of varying, different persuasions – can experience with the injection of potassium chloride? But I don’t think that in any sense of the word it can be described as excruciating,” he said.