ST. LOUIS — Missouri Gov. Jay Nixon has postponed the execution of murderer Allen Nicklasson, scheduled for Oct. 23, because of concerns about the state’s plan to use a lethal dose of propofol, a common surgical anesthetic.

The European Union (EU), which opposes the death penalty, had threatened to cut off supplies of the drug to the U.S. if the execution went forward, which could have had a widespread impact on hospitals.

That seemed to weigh on Nixon, who said his job was “making sure justice is served and public health is protected.” Last week, the governor had said that Europe would not block the executions.

Nixon said Friday that he had directed the Department of Corrections to change the execution protocol to use a different form of lethal drug, and added that Attorney General Chris Koster would request a new execution date for Nicklasson.

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Death by drug injection in Missouri has been stalled by constitutional challenges. The state turned to propofol because drug manufacturers were no longer selling to prison systems the three drugs previously used in the state’s execution protocol. Missouri would have been the first state to use propofol in an execution.

It was not immediately clear where Missouri would turn next for an execution method. Thirteen states use pentobarbital, a drug commonly used to euthanize animals. The drug was used in death chambers as recently as Wednesday in Texas. But the Danish manufacturer of the drug is not allowing distribution of the drug for executions, and many states are looking for other options.

An attorney for Nicklasson filed a motion in the state Supreme Court on Wednesday to halt the execution. The attorney, Jennifer Herndon, of St. Louis, argued that the use of propofol was untested and that Nicklasson’s legal team needed more time to mount a legal challenge to its use.

Nicklasson was convicted of killing a man who stopped to help along Interstate 70 in 1994 after Nicklasson’s car broke down. Nicklasson later killed two more good Samaritans in Arizona.