Story Highlights • Lethal injection is most common method of execution in the U.S

• Some say injection amounts to cruel and unusual punishment

• One inmate caught in the controversy says, "I block it out"

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NASHVILLE, Tennessee (CNN) -- Philip Workman has prepared to die three times before.

Next week, the convicted killer is prepared to face execution for a fourth time, to say goodbye to his daughter again, give away his belongings, and once again eat his final meal.

And he'll wait to see if a court will again step in before the needle pierces his skin. (Watch what Workman thinks of the procedure that will kill him )

Workman, 53, has spent half his life in a death row cell in Nashville's Riverbend Prison, ever since his conviction in 1982 for killing a Memphis police officer in a botched armed robbery at a Wendy's restaurant.

On Sunday, Tennessee prison guards will lock him in an isolated cell with a small window overlooking the prison yard, beginning three days of "Death Watch."

He will be fitted for the drab scrubs he'll wear May 9, when the state is set to inject him with a mixture of drugs that will kill him.

Workman said he doesn't feel much like a person anymore. He has become a pile of legal briefs, appeals, depositions.

And he is angry, sorry, scared and depressed. (Watch how Workman has changed on Death Row )

Of the officer who was killed, Workman says: "Any loss of life is a tragedy."

Too little time, too many errors

Lethal injection has become the most common method of execution in the United States. Last year, 52 of the 53 executions in the country were by injection. Of the 38 states that have the death penalty, 37 states allow lethal injection. (Death by lethal injection)

Thirty years after it was developed, the practice is drawing protest as cruel and unusual punishment, a claim supported by recent medical studies that say the mixture of chemicals used may cause a slow and excruciating death.

The debate over lethal injection has led 11 states, including Tennessee, to issue temporary bans on the process pending further study. Tennessee's ban is set to end shortly before Workman's scheduled execution.

"It almost makes me want to choose the electric chair," Workman said in an interview with CNN. "They are saying in this report that a lot [of prisoners] have suffered, they wouldn't be able to speak. You can't move to say anything. You're frozen."

Saying there were "deficiencies" in Tennessee's lethal injection instruction manual, Gov. Phil Bredesen rescinded it in February and gave the state's commissioner of correction 90 days to write a new one.

Bredesen, who declined CNN's request for an interview, has stayed the executions of four death row inmates, but is allowing Workman's to proceed.

Tennessee's rescinded manual appeared to confuse lethal injection with electrocution. For example, it called for an inmate's head to be shaved, and for officials to have a fire extinguisher, electrode gel, an emergency generator and an electrician present. (Read the recently rescinded manual)

On April 30, the state issued a new set of lethal injection procedures, removing those protocols, but the "cocktail" of lethal drugs remains unchanged. (Read the newly proposed procedures)

Drugs are a "failure"

Lethal injection's three-drug cocktail was proposed in 1977 by an Oklahoma medical examiner and anesthesiologist as a cheaper and more humane alternative to the electric chair. In a recent interview with CNN, Dr. Gary Chapman said the time has come to revisit the cocktail. "It may be time to change it," he said. (Read full story)

Until 2005, it had not been scientifically researched.

In a study published in April by the Public Library of Science -- a nonprofit organization of scientists and doctors -- six scientists spent three years analyzing more than 50 medical examiner reports of North Carolina and California prisoners who had been injected with the short-acting anesthetic thiopental, the paralytic pancuronium bromide and the heart stopper potassium chloride. (Read the entire 2007 study )

The study concluded the drug protocol a "failure" because the prisoners had below acceptable levels of thiopental in their systems indicating they probably suffered immense pain before they died.

"I was shocked that there had been no research on what is being used on humans, when in veterinary medicine, pancuronium is strongly discouraged," said study author Dr. Teresa Zimmers, an assistant professor of surgery at the University of Miami School of Medicine.

She also participated in a 2005 medical review of 49 prisoner toxicology reports from Arizona, Georgia, North Carolina and South Carolina that drew similar conclusions.

Health professionals are barred from participating in executions, so prison personnel without medical expertise often perform injections.

Florida has been studying its lethal injection procedures since it took 34 minutes and two rounds of injections to kill Florida prisoner Angel Diaz in December. His executioner, a prison employee, missed his vein and witnesses described Diaz grimacing. (Read about Diaz's execution )

A moratorium continues in California - home to America's largest death row with 664 inmates - after a federal judge ruled in December that prison personnel are improperly trained and the execution chamber is too dark and poorly designed.

Missouri's lethal injection administrator was revealed in 2006 to be a dyslexic surgeon who had been sued 20 times. The physician admitted he sometimes mixed drugs wrong.

No more visits

An affidavit shows a physician involved in the 2005 and 2007 studies reviewed child killer Robert Coe's autopsy. He was lethally injected in April 2000 in Tennessee.

The doctor concluded Coe "was probably awake, suffocating in silence and felt the searing pain of injection of intravenous potassium chloride."

Workman reads over the article about the studies as if it's some legalese he can't quite comprehend.

"I block it out," he finally says.

He doesn't want anyone to see him anymore. On May 9, his daughter and brother are to stay away, his lawyers, too. He wants people to know he's sorry for robbing that Wendy's and that a police officer died.

Even if he gets another stay, he says he can't endure more of this.

Of his return to Death Watch, he remarks, "I don't just want to visit this time."