The rising use of experimental and untested drugs in the execution of U.S. prisoners reached new heights Monday as Oklahoma officials admitted plans to use a new drug combination to execute two men this month.

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In an email sent to state attorneys, the state said it will be using a combination of midazolam, pancuronium bromide and potassium chloride in the executions, which has never been used before. Similar untested drug combinations have been criticized by human rights campaigners as inhumane and have been documented to cause extreme pain and suffering before death.

Richard Dieter, executive director of the Death Penalty Information Center, told the Guardian that the three-drug combination is similar to what has been used in Florida and Ohio in recent executions.

A similar cocktail of drugs used in January to execute Ohio prisoner Dennis McGuire left him struggling and gasping for air—an ordeal that lasted 26 minutes before McGuire died.

Ohio and other states have turned to compounding pharmacies to custom mix experimental drugs after major foreign drug companies began withholding lethal injection drugs from U.S. prisons in opposition to the death penalty.