In a 101-page order on Saturday, Judge Baker embraced arguments by the eight prisoners whose executions had been scheduled, plus one other death row inmate, that Arkansas’s reliance on midazolam, as an execution drug posed a risk to their constitutional rights. The drug is supposed to render a person unconscious and unable to feel pain during a lethal injection.

“The threat of irreparable harm to the plaintiffs is significant: If midazolam does not adequately anesthetize plaintiffs, or if their executions are ‘botched,’ they will suffer severe pain before they die,” Judge Baker, an appointee of President Barack Obama, wrote. She added that the men had “shown a significant possibility that they will succeed on the merits of their method of execution claims based on midazolam.”

The drug is one of the world’s most popular and versatile sedatives, and at least six states have used it for executions since 2013. Less than two years ago, the United States Supreme Court upheld its use as an execution drug.

But the divided Supreme Court’s opinion in that case, Glossip v. Gross, did little to settle the controversy around midazolam, which was developed in the 1970s as an alternative to Valium and emerged only in recent years as an execution drug. After the Supreme Court’s 5-to-4 ruling, critics of the death penalty continued to argue that the drug lacked the power to render a prisoner sufficiently unconscious before executioners administered drugs that cause pain when stopping a person’s breathing and heartbeat.

The drug has been used for executions that mostly drew little outrage, but it was also part of a handful of executions that went awry. In 2014, for instance, midazolam was part of the drug protocol in Arizona when a man’s execution lasted nearly two hours; the state has since agreed not to use midazolam to carry out death sentences.