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Executions in Ohio, which have been suspended for three years, are back on hold after a federal judge ruled that its lethal injection method may be too painful.

The state wanted to resume killing death-row prisoners next month, using a three-drug protocol that includes the controversial sedative midazolam, which has been involved in several executions that did not go as planned.

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Death row inmates sued, arguing that midazolam does not induce the deep state of unconsciousness needed to shield a prisoner from the pain of the next two drugs: one that paralyzes and one that stops the heart.

Federal magistrate judge Michael Merz ruled that "use of midazolam as the first drug in Ohio’s present three-drug protocol will create a 'substantial risk of serious harm' or an 'objectively intolerable risk of harm'" — the threshold set by the U.S. Supreme Court.

He issued a preliminary injunction that postpones three executions scheduled for February, March and April until the court can take a closer look at the arguments that the method violates the constitutional protection against cruel and unusual punishment.

Bottles of the sedative midazolam. AP

The Ohio attorney general's office said it was reviewing the decision and had no immediate comment. The inmates' attorneys and capital punishment opponents praised the ruling, noting that some other death-penalty states have abandoned midazolam.

"The risks of extraordinary pain and suffering with this protocol are unconstitutional, and this ruling correctly recognizes the problems with midazolam,” said Megan McCracken of the Berkeley Law School Lethal Injection Project.

Ohio has not executed a prisoner since 2014 when rapist-murderer Dennis McGuire took 25 minutes to die and appeared to be gasping for air during the state's first execution using midazolam.

Like many states, Ohio has not been able to find alternative execution chemicals, largely because pharmaceutical companies, under public pressure, have stopped selling them to prisons for lethal injections. Executions hit a 30-year low in the U.S. last year.