States began moving to lethal injection in the 1980s in the belief that powerful sedatives and heart-stopping drugs would replace the violent spectacles with a more clinical affair while limiting, if not eliminating, an inmate's pain.
The total number of U.S. executions has declined — from a peak of 98 in 1999 to 39 last year. Some states have turned away from the death penalty entirely. Many have cases tied up in court. And those that carry on with executions find them increasingly difficult to conduct because of the scarcity of drugs and doubts about how well they work.
European drug makers have stopped selling the lethal chemicals to prisons because they do not want their products used to kill.
At least two recent executions are also raising concerns about the drugs' effectiveness. Last week, Ohio inmate Dennis McGuire took 26 minutes to die by injection, gasping repeatedly as he lay on a gurney with his mouth opening and closing. And on Jan. 9, Oklahoma inmate Michael Lee Wilson's final words were, "I feel my whole body burning."
Missouri threw out its three-drug lethal injection procedure after it could no longer obtain the drugs. State officials altered the method in 2012 to use propofol, which was found in the system of pop star Michael Jackson after he died of an overdose in 2009.
The anti-death penalty European Union threatened to impose export limits on propofol if it were used in an execution, jeopardizing the supply of a common anesthetic needed by hospitals across the nation. In October, Gov. Jay Nixon stayed the execution of serial killer Joseph Paul Franklin and ordered the Missouri Department of Corrections to find a new drug.
Days later, the state announced it had switched to a form of pentobarbital made by a compounding pharmacy. Like other states, Missouri has refused to divulge where the drug comes from or who makes it.