Lawyers for a Missouri death row inmate on Tuesday were seeking to halt his execution over concerns about the state’s secret lethal injection drugs a day after an Oklahoma court stopped two executions there over similar issues.

William Rousan, 57, is scheduled for execution at 12.01am CST on Wednesday. Rousan was convicted of murdering 62-year-old Grace Lewis and her 67-year-old husband, Charles Lewis, in 1993 in a plot to steal the farm couple’s cattle.

Attorneys for Rousan have argued that Missouri’s secret execution drugs could cause undue suffering. The eighth US circuit court of appeals on Monday rejected Rousan’s appeal, and the case was headed to the US supreme court.

The action follows a decision issued on Monday by the Oklahoma supreme court that halted the executions of Clayton Lockett, scheduled for Tuesday, and Charles Warner, scheduled for April 29. The court said the inmates had the right to have an opportunity to challenge the secrecy over the drugs Oklahoma intends to use to put them to death.

Lawyers for death row inmates in several states have raised a series of arguments against the use of compounded drugs for executions. Many states have turned to the lightly regulated compounding pharmacies for supplies because makers of drugs traditionally used in lethal injections have largely stopped making them available for executions.

But the lawyers argue that drugs obtained for lethal injections from compounding pharmacies could lead to undue suffering, which would amount to cruel and unusual punishment in violation of the US constitution. They also say they should have information about the legitimacy of the supplier, and details about the purity and potency of the drugs.

Prison officials have rejected those arguments and have been refusing to reveal where they are getting the drugs.

But Louisiana and Ohio this year have seen executions delayed because of concerns about suffering that might be caused by untraditional drug supplies. The family of one inmate executed in Ohio in January has filed suit against the state because, according to some witnesses, he took an unusually long time to die and appeared to be in pain.

Last year, Missouri started classifying compounding pharmacies as part of its execution team and said the identities of the pharmacies were thus shielded from public disclosure.