The execution protocols used by states in the USA call for three drugs given in this order:

1. A fast acting barbiturate to put the victim to sleep.

2. A curare derivative to cause complete paralysis. This is for the sake of the witnesses, not the victim.

3. Potassium chloride to stop the heart and cause rapid death.

The barbiturate alone would be enough if given in a large enough dose, but the victim may take 15 to 20 minutes to die, and all the while could be twitching and making sounds. So the other two drugs are added to hurry the process along. If given in the wrong sequence, or if the second and third are given too quickly after the first, the victim dies in horrible pain but is not able to move or cry out, so no one knows that's what's happening.