A South Dakota man on death row is petitioning the state to use a lethal injection drug of his choosing, just two weeks from his planned execution date.

Charles Russell Rhines has asked the state to use the same type of drug that was mandated for executions at the time of his conviction, which occurred in 1993 when protocol was to use an ultra-fast-acting drug and a chemical paralytic.

But, the state of South Dakota has signalled its intention to use pentobarbital, which is commonly used to euthanise animals and one that critics say amounts to cruel and unusual punishment when used for executing humans.

Rhines’ lawyers filed a complaint on Tuesday, arguing that the use of pentobarbital is a violation of their client’s due process and right to choose the manner of his execution. At the time of his conviction, stemming from the 1992 death of 22-year-old Donnivan Schaeffer, South Dakota required the use of an ultra-fast-acting drug alongside a paralysing agent — which Pentobarbital is not.

“The issue is should the state follow its law when attempting to carry out the death penalty, and the obvious answer to that is yes, because we’re talking about a process that is kind of the ultimate exercise of state power,” Robert Dunham, the executive director of the Death Penalty Information Centre, told The Independent.

Activists protest the death penalty in Texas Show all 6 1 /6 Activists protest the death penalty in Texas Activists protest the death penalty in Texas Family and friends of Jeff Wood and anti-death penalty activists march from the Texas State Capitol to the Office of Governor Greg Abbott to deliver a petition with over 10,000 signatures asking the governor and the Texas Board of Pardons and Paroles to commute Wood's sentence on August 18, 2016 in Austin, Texas. Jeff Wood is scheduled to be executed by the state of Texas on August 24, 2016 under what is referred to as the "law of parties" for a criminal act he committed on January 2, 1996. The Texas law says that if a person, " acting with intent to promote or assist the commission of the offense, he solicits, encourages, directs, aids, or attempts to aid the other person to commit the offense;" then they are criminally responsible for the conduct of another, as well. Tamir Kalifa/AFP/Getty Images Activists protest the death penalty in Texas Carla Harvey adds her name to a petition with over 10,000 signatures asking Governor Greg Abbott and the Texas Board of Pardons and Paroles to commute Jeff Wood's sentence, during a demonstration outside of the Texas State Capitol on August 18, 2016 in Austin, Texas. Family and friends of Jeff Wood and anti-death penalty activists delivered the petition to the Office of the Governor after the demonstration. Jeff Wood is scheduled to be executed by the state of Texas on August 24, 2016 under what is referred to as the "law of parties" for a criminal act he committed on January 2, 1996. The Texas law says that if a person, " acting with intent to promote or assist the commission of the offense, he solicits, encourages, directs, aids, or attempts to aid the other person to commit the offense;" then they are criminally responsible for the conduct of another, as well. Tamir Kalifa/AFP/Getty Images Activists protest the death penalty in Texas A massive explosion guts Mexico's biggest fireworks market in Mexico City, on December 20, 2016. The explosion killed at least 31 people and injured 72, authorities said. The conflagration in the Mexico City suburb of Tultepec set off a quick-fire series of multicolored blasts that sent a vast cloud of smoke billowing over the capital. Tamir Kalifa/AFP/Getty Images Activists protest the death penalty in Texas Darius Broussard adds his name to a petition with over 10,000 signatures asking Governor Greg Abbott and the Texas Board of Pardons and Paroles to commute Jeff Wood's sentence, during a demonstration outside of the Texas State Capitol on August 18, 2016 in Austin, Texas. Family and friends of Jeff Wood and anti-death penalty activists delivered the petition to the Office of the Governor after the demonstration. Jeff Wood is scheduled to be executed by the state of Texas on August 24, 2016 under what is referred to as the "law of parties" for a criminal act he committed on January 2, 1996. The Texas law says that if a person, " acting with intent to promote or assist the commission of the offense, he solicits, encourages, directs, aids, or attempts to aid the other person to commit the offense;" then they are criminally responsible for the conduct of another, as well. Tamir Kalifa/AFP/Getty Images Activists protest the death penalty in Texas Family and friends of Jeff Wood and anti-death penalty activists march from the Texas State Capitol to the Office of Governor Greg Abbott to deliver a petition with over 10,000 signatures asking the governor and the Texas Board of Pardons and Paroles to commute Wood's sentence on August 18, 2016 in Austin, Texas. Jeff Wood is scheduled to be executed by the state of Texas on August 24, 2016 under what is referred to as the "law of parties" for a criminal act he committed on January 2, 1996. The Texas law says that if a person, " acting with intent to promote or assist the commission of the offense, he solicits, encourages, directs, aids, or attempts to aid the other person to commit the offense;" then they are criminally responsible for the conduct of another, as well. Tamir Kalifa/AFP/Getty Images Activists protest the death penalty in Texas Mark Clements (L), who was imprisoned for 28 years before being released and exonerated, encourages passersby to sign a petition with over 10,000 signatures asking Governor Greg Abbott and the Texas Board of Pardons and Paroles to commute Jeff Wood's sentence, during a demonstration outside of the Texas State Capitol on August 18, 2016 in Austin, Texas. Clements joined family and friends of Jeff Wood and anti-death penalty activists to deliver the petition to the Office of the Governor after the demonstration. Jeff Wood is scheduled to be executed by the state of Texas on August 24, 2016 under what is referred to as the "law of parties" for a criminal act he committed on January 2, 1996. The Texas law says that if a person, " acting with intent to promote or assist the commission of the offense, he solicits, encourages, directs, aids, or attempts to aid the other person to commit the offense;" then they are criminally responsible for the conduct of another, as well. Tamir Kalifa/AFP/Getty Images

Pentobarbital is approved for use in executions in several US states, including Texas and Missouri, as well as in executions conducted by the federal government under a policy reversal implemented by the Trump administration.

But, as the drug has been criticised for the potential pain it might bring to those being executed — South Dakota inmate Rodney Berget, for instance, infamously asked “is it supposed to feel like that?” during his 12 minute execution last year — the question about the source of the drug has also proved elusive.

Mr Dunham said that there are no manufactures in the United States that are approved by the US Food and Drug Administration that have approved the sale of pentobarbital to states for use in executions. That means that states like South Dakota — which have refused to disclose the source of their execution drugs — and others could potentially be using drugs that are old, or even improperly stored.

“Their policy is that it is a state secret,” Mr Dunham said. “It reminds me of a caution from president [Ronald] Regan, about how the scariest thing in the world is somebody coming up to you and saying, ‘trust me, I’m the government.’”

He continued: “‘Trust me I’m the government followed by, ‘I’m not going to tell you that’ suggests there is something to hide.”

In the court filing, Rhines and his lawyers have also asked for disclosure of the source of drugs that could be used in his execution. The state is unlikely to provide that information.

South Dakota assistant attorney general Paul Swedlund said in an October letter that the state Department of Corrections would not disclose the source of the drugs, but said that no drug would be used that had passed its expiration date.

“Obviously, drugs used in the United States must be produced in an FDA-approved facility,” Mr Swedlund wrote.