The firing squad is the only appropriate method of execution for a condemned Georgia prisoner – even though it's not permitted under state law – because the state's lethal injection drug could cause him to suffer more than the Constitution allows, his lawyers say.

JW Ledford Jr is scheduled to die on Tuesday. He was convicted of murder in the January 1992 stabbing death of his neighbour, 73-year-old Dr Harry Johnston, near his home in Murray County, in northwest Georgia.

Mr Ledford, 45, suffers from chronic nerve pain that has been treated with increasing doses of the prescription drug gabapentin for more than a decade, his lawyers said in a federal lawsuit filed on Thursday. They cited experts who say long-term exposure to gabapentin alters brain chemistry, making pentobarbital unreliable to render him unconscious and devoid of sensation or feeling.

"Accordingly, there is a substantial risk that Mr Ledford will be aware and in agony as the pentobarbital attacks his respiratory system, depriving his brain, heart, and lungs of oxygen as he drowns in his own saliva," the lawsuit says.

That would violate the prohibition on cruel and unusual punishment in the Eighth Amendment of the US Constitution, Mr Ledford's lawyers argue.

US District Judge Steve Jones on Friday dismissed the lawsuit, and Mr Ledford's lawyers filed notice of their intent to appeal to the 11th US Circuit Court of Appeals.

State lawyers had called Mr Ledford's lawsuit “manipulative”, and said the experts cited by his lawyers made dubious claims based on speculation and that they failed to show that pentobarbital is “sure or very likely” to cause him “a substantial risk of serious harm”, as required by US Supreme Court precedent.

State lawyers also cited their own expert, Dr Jacqueline Martin, chief medical examiner for the Georgia Bureau of Investigation, who said the amount of pentobarbital used by the state is “more than sufficient” to carry out the execution without causing Ledford pain, despite his use of gabapentin.

The US Supreme Court has said that when claiming an execution method violates the Eighth Amendment, an inmate must propose a “known and available” method of execution.

There is no alternative method of lethal injection available to the state, since the drugs used in executions have become harder for states to obtain because manufacturers have prohibited their use for capital punishment, the lawsuit says. So Mr Ledford's lawyers proposed using a firing squad, which the Supreme Court has already declared constitutional. Georgia already has the skilled personnel, weapons and ammunition needed to carry out an execution by firing squad, Mr Ledford's lawyers argued.

Three states – Mississippi, Oklahoma and Utah – allow for a firing squad as a backup if lethal injection drugs aren't available, said Robert Dunham, executive director of the Death Penalty Information Centre, which compiles capital punishment statistics.

Georgia's lawyers noted that the 11th Circuit ruled this week in an Alabama case that the state must be able to implement the proposed alternative “relatively easily and reasonably quickly.” State courts and the Georgia Department of Corrections cannot change Georgia law, which only allows execution by lethal injection, so Mr Ledford has failed to meet that standard, the state argues.

If Mr Ledford “really thought the firing squad was a reasonable alternative he could have alerted the State years, instead of five days, before his execution,” state lawyers said in a court filing.

Ledford's lawyers argue that because Georgia law only allows for lethal injection, they are effectively prevented from meeting the burden imposed by the Supreme Court when challenging an execution protocol on the grounds that it is cruel and unusual.

Mr Jones wrote in his order that Mr Ledford's lawyers had failed to demonstrate that execution by pentobarbital was “sure or very likely” to cause him extreme pain. He also noted that Mr Ledford waited until just a few days before his execution date to file a challenge, which he said “suggests dilatory tactics meant to forestall the state's ability to exact its lawful punishment.”

Mr Ledford's attorneys also have asked the State Board of Pardons and Paroles to spare his life, citing a rough childhood, substance abuse from an early age and his intellectual disability.