US District Judge Jeremy Fogel in San Jose, California, said he was concerned inmates were conscious and undergoing extreme pain once a paralysing agent and then heart stopping medication began coursing through their veins.
But he declined to immediately postpone the execution next Tuesday of Michael Morales, instead he ordered the state to either have an expert present to ensure he's unconscious from a sedative or replace a three-drug combination with a lethal dose of barbiturate.
If the state declines to choose either option, Judge Fogel said he would stay Morales’ execution and have hearings on whether it is cruel and unusual punishment - a violation of the US Constitution.
Morales, 46, of Stockton, was convicted in 1983 of murdering Terri Winchell, 17, who was found beaten, stabbed and raped in a secluded vineyard.
Lawyers for Morales alleged a mistake in the sedation process might mean he would appear unconscious, but internally would feel excruciating pain once the paralysing and death agents were administered.
The state switched to lethal injection after a California judge ruled that San Quentin State Prison's gas chamber was cruel and unusual punishment.