Story highlights J.W. Ledford Jr. has been on death row 25 years

His lawyers say medication has changed his brain chemistry

(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.

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The state also questioned the timing of Ledford's arguments. He is scheduled to be executed on Tuesday.