A handful of states, including Texas and Georgia, along with the federal system, have most recently used pentobarbital sodium alone in their execution protocols, according to the Death Penalty Information Center.

The Democrats raised questions about the lethal drug pentobarbital sodium that will likely be used amid problems reported at some manufacturers.

Another possible cause could be that the pharmaceutical solvent used to dissolve pentobarbital sodium during the compounding process either absorbed some water or evaporated during preparation, he wrote.

Another possible cause could be that the pharmaceutical solvent used to dissolve pentobarbital sodium during the compounding process could have either absorbed some water or evaporated during preparation, he wrote.

"The department was required to develop a new protocol because we have limited supplies of pentobarbital sodium that will expire at the end of November 2013," Cash said.