An Oklahoma pharmacy has agreed not to supply compounded pentobarbital for an upcoming execution in Missouri, following a lawsuit filed by the convict arguing the substance is likely to cause “ultimately inhumane pain.”

The lawsuit was filed by Michael Taylor, whose execution is scheduled for February 26 and who alleged that Missouri could obtain the lethal drug - pentobarbital - from the Apothecary Shoppe of Tulsa. 

A federal judge ruled last week that the pharmacy must suspend the supply of the drug to Missouri until further review. The Apothecary Shoppe chose to settle the case out of court. 

"The Apothecary Shoppe has agreed that it will not prepare or provide pentobarbital or any other drug for use in the execution of Michael Taylor," Carrie Apfel, Taylor's attorney, said in a written statement, according to St Louis Public Radio. She also said the pharmacy had not sold any drugs to the Department of Corrections for this execution yet. 

The settlement comes as US states which implement capital punishment face a growing shortage of lethal substances, following the 2011 decision by the EU to stop altogether the sale and export to the US of drugs that could potentially be used for executions. 

The move has forced some of the states still to switch from a three-drug cocktail used in the executions to just one drug – pentobarbital. 

The only licensed manufacturer of pentobarbital, Illinois-based Akorn Inc., purchased exclusive rights to the drug from a Danish company two years ago. The condition for the transaction was that the American firm agreed not to sell the substance for lethal injections. 

Thus, the only option left is obtaining the substance from compounding pharmacies. In his lawsuit, Taylor, according to AP, gives several recent executions as examples of compounded pentobarbital causing "severe, unnecessary, lingering and ultimately inhumane pain."

Revamped lethal injection room at San Quentin State Prison supplied by the California Department of Corrections and Rehabilitation.(Reuters / Handout)

One such example is the execution of Michael Lee Wilson, 38, carried out in Oklahoma January 9. 

"I feel my whole body burning," Wilson said 20 seconds after he started receiving his lethal injection. 

Taylor alleged in his lawsuit that Wilson’s words described "a sensation consistent with receipt of contaminated pentobarbital." 

The lawsuit also questions whether the Tulsa pharmacy can legally produce compounded pentobarbital and deliver it to other states. 

It’s not yet clear how Apothecary Shoppe’s decision not to supply pentobarbital to Missouri will impact the scheduled execution of Michael Taylor, who pleaded guilty to abducting, raping and stabbing to death a 15-year-old Kansas City girl in 1989. 

"The Department of Corrections is prepared to carry out that execution," Missouri Gov. Jay Nixon said at last week’s news conference, according to AP. 

A high-ranking state official revealed last month that the state possessed another lethal drug - midazolam. 

Midazolam’s use has been deemed highly controversial, following its use in the January 16 execution of Dennis McGuire, a convicted rapist and murderer, in Ohio. The execution lasted for 25 minutes and was marked by the prisoner’s prolonged gasping for air. 

The Parliamentary Assembly of the Council of Europe reacted to the execution by describing it as “cruel and inhuman” and calling on Americans to protest against “such atrocities.” 

New lethal substances are likely leading to longer and more painful executions, according to a recentstudy by researchers affiliated with Britain’s Guardian newspaper, who examined three years’ worth of executions carried out in Texas, the state that fulfills more of its death penalty sentences than any other in the US. 

Facing a shortage of lethal drugs, some states have been pondering switching to older methods of execution. Virginia has been considering legislation which would force a prisoner to acceptelectrocution in the absence of the drugs needed for a lethal injection. 

Missouri Attorney General Chris Koster has spoken of the return of the gas chamber in the state, while Missouri State Rep. Rick Brattin in January proposed making firing squads an option for executions.

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