(CNN) -- A Georgia inmate is asking to be executed by a firing squad because he says lethal injection would be too painful for him.
Convicted murderer J.W. Ledford Jr. takes a pain medication, gabapentin, that changed his brain chemistry so much the lethal injection drug pentobarbital might not make him unconscious and would cause him "to suffer an excruciating death," according to documents filed by his lawyer in US District Court.
"Mr. Ledford proposes that the firing squad is a readily implemented and more reliable alternative method of execution that would eliminate the risks posed to him by lethal injection," his lawyers said in court papers filed Thursday.
The Georgia attorney general's office replied Friday there was no proof a firing squad would be less painful and contended there was "no substantial risk" he would suffer severe pain in a Georgia execution by lethal injection.
The state also questioned the timing of Ledford's arguments. He is scheduled to be executed on Tuesday.
"Plaintiff has waited until the eve of his execution to suddenly claim that he has been treated for pain with medication that will allegedly interfere with his execution, ..." the state's lawyers wrote. "If plaintiff really thought the firing squad was a reasonable alternative he could have alerted the State years, instead of 5 days, before his execution."
Ledford's lawyers concede they don't expect their legal strategy to succeed.
Legal precedent only allows him to suggest an alternative form of execution allowed by Georgia, but the state only authorizes lethal injection, the lawyers wrote. Ledford's "dilemma illustrates why this standard is unworkable," the lawyers said.
The lawyers ask that the judge grant a declaratory judgment that Georgia's use of lethal injection violates Ledford's eighth amendment rights, grant an injunction preventing the state from proceeding with an execution using pentobarbital and prevent the state from discontinuing Ledford's use of his pain medication.
Killed neighbor in 1992
Ledford has been on death row 25 years. He was convicted of murder and other crimes in the death of an elderly neighbor, Dr. Harry Buchanan Johnston Jr., in Murray County, Georgia, on January 31, 1992.
He's suffered nerve pain in his back, hips and legs for at least 10 years, his lawyers say, and has been treated with gabapentin, commonly marketed as Neurontin, which is often used to treat epilepsy and neuropathic pain.
His lawyers say gabapentin alters a person's brain chemistry by making brain receptors more receptive to the drug and less receptive to other drugs, including pentobarbital.
In the court documents, the Georgia attorney general's office calls that statement speculative and says the 5,000 mg of pentobarbital used in Georgia executions is more than enough to prevent Ledford from feeling pain.
Controversy over midazolam
Lethal injection is the most common form of execution in the United States, but pentobarbital is not the most controversial element.
Death row inmates nationwide have challenged the use of midazolam in executions.
Critics say it's a sedative but not a painkilling anesthetic, meaning the condemned would feel tortuous pain from the drugs that come next.
Some inmates executed with midazolam convulsed and gasped after receiving injections. For instance, it took almost two hours for an Arizona inmate to die in 2014, with one witness saying it was like a "fish on shore gulping for air."
Some states turned to using midazolam because manufacturers of other drugs said they didn't want their products used for the death penalty.
However, the US Supreme Court upheld the use of midazolam in a 2015 ruling.
3 states allow firing squads
Mississippi, Oklahoma and Utah allow a firing squad to be used for executions, though lethal injection is the primary method in all three states, according to the Death Penalty Information Center website.
Ledford's lawyers concentrate on Utah, saying the state has executed three people by firing squad since 1976, most recently in July 2010.
"In Utah's most recent execution by firing squad, the inmate was seated in a chair set up between stacked sandbags to prevent the bullets from ricocheting," his lawyers wrote. "A target was pinned over the inmate's heart. Five shooters set up at a distance of 21 feet from the inmate, armed with .30-caliber Winchester rifles. One rifle was loaded with blanks so that no one knew which officer killed the inmate. The inmate was pronounced dead two minutes after he was shot."
A firing squad has less chance for "operator error" and would reduce Ledford's chance of experiencing severe pain, his lawyers wrote.