Questions were raised about Ohio's protocol after Dennis McGuire's execution in January took 25 minutes. Witnesses said McGuire appeared to be gasping for breath for much of that time.
U.S. District Judge Gregory Frost ordered a moratorium in May, which would have expired this month. On Friday, he extended it until Jan. 15.
After the McGuire execution, the Department of Rehabilitation and Correction changed its protocol. The doses of midazolam and hydromorphone were doubled to 50 mg each.
But last month, when Joseph Wood III was executed in Arizona, using those dosages, he was not pronounced dead until two hours after the procedure began.
There have been other problematic executions this year. In Oklahoma, Clayton Lockett died of a heart attack after prison officials ordered his execution halted.
JoEllen Smith, a DRC spokeswoman, said the state "remains committed to carrying out executions in a humane and lawful manner." She said Ohio will obey Frost's order.
Four executions scheduled between Sept. 18 and January 7 must be given new dates by the state Supreme Court.