Potassium chloride
However, other doctors say that lethal injection is less painful than other ways of executing people, like using an electric chair or a gas chamber.  They also say that if the three drugs are given correctly, lethal injection should be pain-free.  Because doctors are supposed to "do no harm", many executions are now done by people who are not doctors.  Supporters of lethal injection say that mistakes, not the drugs themselves, are what cause pain.