Amid claims that its drug supply has expired, Texas is set to execute its first prisoner since the state’s attorney general decided that it does not have to reveal the source of its lethal injection drugs.

Citing security concerns for pharmacies, Greg Abbott – the Republican candidate for governor in this November’s election – reversed course in May after previously compelling the Texas department of criminal justice to disclose details about its drug suppliers.

But lawyers for Willie Trottie, who is scheduled to be put to death on Wednesday at 6pm CT unless last-minute legal appeals succeed, have raised questions about whether Texas’s supply is still current and claim that recent botched executions have added to doubts about the efficacy and legality of the lethal injection process in general.

Last month, Arizona inmate Joseph Wood took nearly two hours to die on the gurney after being repeatedly injected with lethal drugs, becoming the latest in a string of botched executions in the US this year.

That has prompted fresh scrutiny of how states carry out executions and raised concerns about the use of the sedative midazolam. Texas is believed to have a stock of midazolam, but its execution protocol calls for only one drug, pentobarbital, and state officials argue that none of its 33 executions since it adopted the single-drug method in 2012 went obviously awry.

A European-led boycott has led to a shortage of manufactured pentobarbital previously used in US executions; as a result, many states have turned to compounding pharmacies to secure the drugs they require, without making the names of those pharmacies public.

“The mechanism for putting a person to death in this country is on the verge of collapse,” Trottie’s lawyers write in a court motion for a temporary restraining order that was denied last week. “Frighteningly, Texas is pursuing the path of secrecy in the midst of these deeply troubling events.”

They argue that when Texas provided them with test results assessing the quality of its stock of compounded pentobarbital, the results were four months old and suggest the state did not conduct tests for sterility. Trottie’s lawyers claim that as a result the quality-assurance information is meaningless for his imminent execution and could indicate that Texas plans to use drugs that are past their expiry date.

“Defendants’ behaviour has only become more secretive, and its intentionally obstructionist nature further underscores Mr Trottie’s claim that his due process rights have been and are being violated,” they write.

“What little information has been disclosed indicates only the possibility that the drug they intend to use to carry out Mr Trottie’s execution is beyond its use date. The steadily increasing numbers of botched executions nationwide provides unfortunate evidence in support of Mr Trottie’s assertions that there is a substantial risk that he will suffer excruciating pain at the time of his execution, in violation of the eight amendment.”

In a brief to the federal fifth circuit appeals court filed on Tuesday, Trottie’s lawyers claim that since the drugs were tested on 17 March this year, they must have been compounded before that date and so are in all probability past their use-by date and possibly contaminated.

Texas prison officials insist that their supply is suitable. “The drugs have been tested for potency and defect. The drugs have a potency of 108% and were found to have no defects. The pentobarbital is not expired and has a use by date of September 2014,” Jason Clark, a spokesman for the Texas department of criminal justice, said in an email.

Trottie’s lawyers also contend that he did not receive adequate legal representation at his original trial and that the prosecution withheld a witness statement about his mental state that might have caused a jury to hand down only a life sentence. Trottie – who turned 45 on Monday – was convicted for shooting to death Barbara Canada, his former common-law wife, and her brother, Titus, during a gun battle at the family home in Houston in 1993.

Another Texas execution is scheduled for 17 September, when Lisa Coleman is to receive a lethal injection for her role in the starvation death of a child. The state is on pace to execute 11 inmates this year. That would be below last year’s total of 16 and would represent the lowest tally since 1996, when only three prisoners were executed because of a challenge to a law passed by the Texas legislature that modified the appeals process; the following year, 37 inmates were put to death.

While the numbers appear to be trending downwards long-term and it is currently level with Florida and Missouri on seven executions this year, Texas is not about to relinquish its title as the nation’s most active death penalty state.

Five inmates with looming dates received long-term stays this year and Texas has already scheduled six executions between January and March in 2015.