Potassium chloride
The use of potassium chloride in lethal injections is controversial.  For example, one scientist who studied the three-drug combination called potassium chloride "extremely painful."   United States Supreme Court Justice Stephen Breyer used her research to argue that lethal injection is cruel and unusual punishment.  Other doctors agree that if the drugs are not given correctly, a prisoner could experience "a great deal of pain" or "excruciating [terribly painful] suffering."