St. Louis ? The death penalty in the United States may be hitting a roadblock: the Hippocratic Oath.

Condemned inmates in three states have successfully challenged lethal injection as cruel and unusual.

Meanwhile, states are vigorously defending their use of a procedure that seems more humane than alternatives.

For the first time, judges have sided with inmates in ruling that lethal injection has the potential to be unconstitutionally cruel – that without doctors present, the procedure could be inhumane.

The problem is that few doctors are willing to do it. Ultimately, this may be the undoing of lethal injection.

Lethal injection is the primary execution method of 37 of the 38 states with the death penalty. Seventeen of the 37 states require that a doctor participate.

Missouri does not, but has used a physician. States not using doctors rely on other medical personnel, such as nurses and EMTs.