The US Supreme Court is due to review the use of lethal injection drugs following several bungled executions that saw inmates gasping for air during prolonged killings.
The court will revisit the 2008 "Baze vs. Daze" ruling that deemed lethal injection does not violate the Eighth Amendment to the US Constitution, which prohibits "cruel and unusual punishment."
The court could ban the use of the controversial midazolam sedative, which was used in three botched executions in the United States last year and is not approved by the US Food and Drug Administration to be used as an anesthetic.
The case was brought by four Oklahoma inmates scheduled to be killed on death row, but one was killed before this week's decision to stay all executions using the disputed drug in the state.
Drug companies have refused to produce drugs to be used for executions and the 32 US states where capital punishment remains legal are facing shortages.
Some facts about the case and what it could mean for lethal injection in the United States:
Q: What has the Supreme Court agreed to consider?
Strictly speaking, the court on January 23 agreed to consider the use of midazolam in lethal injections in Oklahoma.
Last April, Oklahoma death row inmate Clayton Lockett took an agonizing 43 minutes to die and could be seen writhing in pain during the prologed execution.
Now, the court has agreed to consider -- likely at the end of April -- whether it can authorize the use of the lethal injection cocktail.
The court will decide whether the drug fully sedates the inmate to ensure he does not feel pain from the other drugs used to paralyze and kill him.
The nine judges will also consider whether a death row inmate has to prove that a better alternative execution method is available, even if the existing lethal injection method is deemed unconstitutional.
Q: Why is midazolam used?
This sedative is used in Oklahoma and other states as an anesthetic that is meant to render the inmate completely unconscious before he is injected with lethal drugs.
But the drug is not 100 percent reliable and the US Food and Drug administration has not approved its use as an anesthetic.
Experts say that if the dosage is incorrect, the inmate can feel excruciating pain after being put into a state of paralysis, which opponents have said violates Eighth Amendment rights.
Q: What precedents exist?
Midazolam has been used in three executions in 2014 where inmates are believed to have suffered, including in Lockett's prolonged death.
On January 16, 2014, Ohio inmate Dennis McGuire took 26 minutes to die, and Arizona death row convict Joseph Wood took 117 minutes to die on July 23.
Lethal injection executions are expected to take 10 minutes, and in all three cases, the men could be seen gasping for air.
Q: What's at stake?
The US Supreme Court can at minimum ban all lethal injections using midazolam, if the court finds the drug causes pain that it deems unconstitutional.
But it could go even further by reversing the 2008 ruling that lethal injection was constitutional in a chaotic landscape where each state uses different drugs produced by compounding pharmacies.
Q: Which US states use midazolam?
Florida and Oklahoma use midazolam combined with two other drugs, while Ohio and Arizona have used it in combination with only one other substance. Missouri has administered the drug as a sedative before the official lethal injection protocol. These five states are responsible for half of all 41 executions carried out so far this year and last.
But other states -- including Alabama, Kentucky, Louisiana and Virginia -- are currently considering using midazolam in lethal injection executions.
Q: Who are the plaintiffs in the current case?
The case was brought forward after Lockett's botched killing by four other death row inmates: plaintiffs Charles Warner, Richard Glossip, Benjamin Cole and John Grant.
The case was originally dubbed "Warner vs. Gross," but Warner was executed on January 15 after the Supreme Court rejected his last-minute appeal for clemency.
The case is now referred to as "Glossip vs. Gross," and the executions of the three plaintiffs have been stayed while the court considers the issue.
Q: Why was one of the four plaintiffs executed?
Only four of the nine Supreme Court justices voted to stay Warner's execution. It requires five votes, but according to court protocol, only four votes are required to consider a case and the court has now agreed to take up "Glossip vs. Gross." Likely with the four same votes that would have granted Warner's stay.
Not everyone agrees with this Supreme Court rule, however. Hofstra University constitutional law professor Eric Freedman said the court should "stop permitting executions when four justices object."