Amber McLaughlin, convicted of murder, on Tuesday became the first openly transgender person to be executed in the United States. The 49-year-old was reportedly accompanied by a spiritual adviser when she was given a fatal dose of pentobarbital. The Missouri inmate's clemency request was denied by Gov. Mike Parson.

McLaughlin, who transitioned into a woman about three years ago at a prison in Potosi, was convicted of killing a former girlfriend in 2003, as per CBS News. The woman's body was dumped near a Mississippi river.

"I am a loving and caring person," McLaughlin wrote in her final statement while apologizing for her acts, the outlet reported.

What do we know about pentobarbital?

Pentobarbital, also commonly used to put down pets, is a sedative that curbs the activity of the brain and nervous system. The compound also has a history of being used in a cocktail of drugs during executions.

Pentobarbital was used alongside potassium chloride and vecuronium bromide in the 2014 execution of convicted murderer Michael Lee Wilson in Oklahoma. "I feel my whole body burning," he reportedly said while the injections were being administered.

When death row inmates are given the cocktail lethal injection, it is unclear which element kills the individual. All three drugs (pentobarbital, pancuronium bromide and potassium chloride) can have lethal effects, anesthesiology expert Dr. Scott Segal of Tufts Medical Center told The New York Times.

According to the Death Penalty Information Center, at least 14 states in the U.S. have already used pentobarbital to carry out executions. These states include Texas, Florida, Alabama, Washington, Ohio, Georgia and Arizona. While Colorado uses pentobarbital as a backup drug, states like Kentucky, North Carolina, Montana, Louisiana and Tennessee are yet to use it.

Pentobarbital has limited medical application but has previously been used for physician-assisted suicide. It is also used to artificially induce a coma in patients who have brain damage. This "allows the brain to use more energy and oxygen to repair itself," Dr. Segal told The New York Times.

Data from the Death Penalty Information Center suggests that more than 1,500 individuals have been executed in the U.S. since the mid-1970s.

Execution
Carlton Gary, also infamously known as the Stocking Strangler, was set to die by lethal injection on Thursday. This photo shows, the 'death chamber' at the Texas Department of Criminal Justice Huntsville Unit in Huntsville, Texas, Feb. 29, 2000. Getty Images/ PAUL BUCK