A judge has blocked Arkansas officials from using one of three drugs in a series of planned executions this month.
Judge Wendell Griffen of the Sixth Judicial Circuit in Little Rock, issued the temporary restraining order in response to a petition from McKesson Medical Surgical Inc.
The drug company said the vercuronium bromide had been sold to the state for medical purposes - not capital punishment.
The drug is used by Arkansas along with potassium chloride and midazolam. The midazolam is to sedate the inmate before the other two drugs paralyse the lungs and stop the heart.
Another hearing on the matter has been set for Tuesday.
The state planned to execute seven prisoners by lethal injection over 11 days, beginning on Monday.
They would be the first executions in Arkansas in 12 years but lawyers for the inmates have said the rush to kill is unconstitutional and reckless, risking mistakes and traumatising prison staff.
Arkansas governor Asa Hutchinson, however, is in a hurry because the state's supply of midazolam expires at the end of April and the chances of getting a substitute are slim.
On Thursday, the pharmaceutical companies behind the other two ingredients of the lethal injection joined a legal bid to stop their drugs from being used in executions .
In a court filing, Fresenius Kabi USA and West-Ward Pharmaceuticals said they object to their products being used by the state of Arkansas for lethal injections as it "runs counter to the manufacturers' mission to save and enhance patients' lives".
Meanwhile, the Arkansas Supreme Court granted an emergency stay of execution for Bruce Ward, one of the inmates who was due to be executed on Monday.
The 60-year-old was convicted of murdering a female convenience store worker and was sentenced to death 17 years ago but his lawyers have said he is schizophrenic and doesn't understand what is to happen to him.
Representing Ward, Scott Braden said: "We're grateful that the Arkansas Supreme Court has issued a stay of execution...so that they may consider the serious questions presented about his sanity".
Another case is also before US District Judge Kristine Baker which argues that the busy execution schedule could lead to undue pain and suffering.
Ms Baker is also considering the case of Fresenius Kabi USA and West-Ward Pharmaceuticals.