For decades, the typical lethal injection used a so-called cocktail of an anesthetic, a paralytic agent and potassium chloride. The method was adopted in the '80s as a less jarring alternative to the electric chair.
But supplies of the drugs have dwindled over the past few years, leading states to try different methods: from single-drug injections to different anesthetics and barbiturates.
The drugs that states have used instead have come under increased scrutiny after a handful of executions where the prisoner seemed to suffer. The issue even made it to the Supreme Court where it was ruled the drug did not violate the Constitution.
That decision centered on Oklahoma's use of the drug midazolam — the anesthetic used in the botched execution of Clayton Lockett in 2014.
The court ultimately decided the drug can still be used in executions because it didn't cause Lockett's apparent suffering. But four justices dissented, and some even called the constitutionality of the death penalty into question.
Justice Stephen Breyer argued executions could constitute cruel and unusual punishment, writing, "I believe it highly likely that the death penalty violates the Eighth Amendment."
Justice Sonia Sotomayor emphasized the uncertain position drug shortages have left states in, too: "The questions before us are especially important now, given States' increasing reliance on new and scientifically untested methods of execution."
The alternatives to lethal injection, from firing squads to gas chambers to the electric chair, are broadly seen as more brutal, and their use could bring more constitutional scrutiny from the court.
The shortage of drugs is also mirrored by shrinking public support. A majority of Americans still approve of the death penalty, but that support has steadily decreased over the past two decades: down to 56 percent in favor in 2015, from 78 percent in 1996, according to Pew Research Center polls.
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Death penalty, execution methods
How drug shortages will shape the death penalty debate
FILE - This July 25, 2014 file photo shows bottles of the sedative midazolam at a hospital pharmacy in Oklahoma City. Exactly one year after a botched lethal injection, attorneys for other Oklahoma death row inmates were set to ask the U.S. Supreme Court Wednesday, April 29, 2015 to outlaw a sedative used in the procedure â a ruling that could force several states to either find new execution drugs or change the way they put prisoners to death. (AP Photo/Sue Ogrocki, File)
In this Oct. 9, 2014 file photo, Department of Corrections officials are pictured in the witness room at right, outside the newly renovated death chamber at the Oklahoma State Penitentiary in McAlester, Okla. With executions in Oklahoma on hold amid a constitutional review of its lethal injection formula, Republican legislators are pushing to make Oklahoma the first state in the nation to allow the use of nitrogen gas to execute death row inmates. (AP Photo/Sue Ogrocki, File)
An unidentified Arizona Corrections Officer adjusts the straps on the gurney used for lethal injections at the Florence Death House at the Arizona State Prison at Florence (Ariz.) in this undated photo provided by the Arizona Department of Corrections. On Tuesday, Sept. 2, 2003, a federal appeals court overturned more than 80 death sentences in Arizona because a judge, instead of a jury, handed them down. Death sentences in Idaho and Montana also were affected. (AP Photo/Arizona Department of Corrections)
File - In this April 12, 2005 file photo is the death chamber at the Missouri Correctional Center in Bonne Terre, Mo. The Associated Press and four other news organizations filed a lawsuit Thursday, May 15, 2014 challenging the secret way in which Missouri obtains the drugs it uses in lethal injections, arguing the state's actions prohibit public oversight of the death penalty. The suit asks the state's department of corrections to disclose where it purchases drugs used to carry out executions along with details about the composition and quality of those drugs. (AP Photo/James A. Finley, File)
FILE - In this Nov. 2005 file photo, public information director Larry Greene is shown in the death chamber at the Southern Ohio Correctional Facility in Lucasville, Ohio. Ohio prison officials said Friday, Oct. 4, 2013, they are keeping their primary lethal injection drug in place despite the state's supply expiring, but they've added a second drug option for executioners to address the shortage. Prisons spokeswoman JoEllen Smith said the powerful sedative pentobarbital will remain Ohio's primary method of administering the death penalty. A policy posted to the prisons department's website listed a combination of midazolam and hydromorphone as an alternative if sufficient pentobarbital isn't available or if the existing supply "is deemed unusable" by the medical team. (AP Photo/Kiichiro Sato, File)
393846 06: A gurney and a electric chair sit in the death chamber of the Southern Ohio Correctional Facility August 29, 2001 in Lucasville, Ohio. The state of Ohio is one of the few states that still uses the electric chair, and it gives death row inmates a choice between death by the electric chair or by lethal injection. John W. Byrd, who will be executed on September 12, 2001, has stated that he will choose the electric chair. (Photo by Mike Simons/Getty Images)
393846 05: A view of the death chamber from the witness room at the Southern Ohio Correctional Facility shows an electric chair and gurney August 29, 2001 in Lucasville, Ohio. The state of Ohio is one of the few states that still uses the electric chair, and it gives death row inmates a choice between death by the electric chair or by lethal injection. John W. Byrd, who will be executed on September 12, 2001, has stated that he will choose the electric chair. (Photo by Mike Simons/Getty Images)
FILE - This is an undated file photo of the electric chair at the Tennessee State prison in Nashville. First used by New York State in 1890, it was used throughout the 20th century to execute hundreds and is still an option in eight states. Since 1976, 158 inmates have been executed by electrocution. It was considered humane on its introduction but resulted in many horrific executions over the years. (AP Photo, File)
This is a 1996 photo of Yellow Mama, Alabama's electric chair at Holman Prison in Atmore, Ala. Alabama Gov. Don Siegelman and Attorney General Bill Pryor want lethal injections legalized in Alabama, but only as a precaution in case the courts declare the electric chair unconstitutional. (AP Photo)
This undated photo provided by the Virginia Department of Corrections shows an electric chair. Larry Bill Elliott is scheduled to be executed by choice of electrocution Tuesday, Nov. 17, 2009 for the January 2001 shooting deaths of 25-year-old Dana Thrall and 30-year-old Robert Finch. (AP Photo/Virginia Department of Corrections)
This photo taken May 16, 2013, shows an electric chair on exhibit at the Texas Prison Museum in Huntsville, Texas. Between 1924 and 1964, 361 men died in the electric chair. Since the first execution by lethal injection in Texas in 1982 the state has executed 499 prisoners. (AP Photo/Pat Sullivan)
This is the interior of the gas chamber at the Nevada State Penitentiary in Carson City, Nev., seen 1936. (AP Photo)
FILE - In this June 18, 2010, file photo, the firing squad execution chamber at the Utah State Prison in Draper, Utah, is shown. In the wake of a bungled execution in Oklahoma last month, a Utah lawmaker wants to resurrect firing squads as a method of execution in his state. Rep. Paul Ray, a Republican from Clearfield, says firing squads would be a quick and humane way to put someone to death as lawsuits and drug shortages have hampered lethal injections in recent years. Ray plans to introduce his proposal during Utahâs next legislative session in January. Utah stopped allowing death-row inmates to choose execution by firing squad after 2004. Several inmates sentenced before that time have opted for firing squad executions but are appealing their sentences. Utah last used the method in 2010, when a firing squad of five police officers with .30-caliber Winchester rifles executed Ronnie Lee Gardner. (AP Photo/Trent Nelson, Pool, File)
The execution chamber at the Utah State Prison after Ronnie Lee Gardner was executed by firing squad Friday, June 18, 2010. Four bullet holes are visible in the wood panel behind the chair. Gardner was convicted of aggravated murder, a capital felony, in 1985. (AP Photo/Trent Nelson - Pool)
The execution chamber at the Utah State Prison is seen after Ronnie Lee Gardner was executed by firing squad Friday, June 18, 2010 in Draper Utah. Four bullet holes are visible in the wood panel behind the chair. Gardner was convicted of aggravated murder, a capital felony, in 1985.(AP Photo/Trent Nelson/Pool)
The execution chamber at the Utah State Prison after Ronnie Lee Gardner was executed by firing squad Friday, June 18, 2010. The bullet holes are visible in the wood panel behind the chair. Gardner was convicted of aggravated murder, a capital felony, in 1985. (AP Photo/Trent Nelson - Pool)
A huge crowd of over 15, 000 people gathers around a scaffold to witness the public hanging of 22-year old Rainey Bethea August 14, 1936 in Owensboro, Kentucky. Public outrage over the manner of execution made Bethea's death the last public hanging in the country. (Photo by Newsmakers)