Originally Posted by Bryan G

Correct me if I am wrong but lethal injection is not painful. They put you to sleep first, then stop your breathing, then your heart.

The intravenous injection is usually a sequence of drugs given in a set sequence, designed to first induce unconsciousness followed by death through paralysis of respiratory muscles and/or by cardiac arrest through depolarization of cardiac muscle cells. The execution of the condemned in most states involves three separate injections (in sequential order):

Sodium thiopental: ultra-short action barbiturate, an anaesthetic agent capable of rendering the prisoner unconscious in a few seconds.

Pancuronium: non-depolarizing muscle relaxant, causes complete, fast and sustained paralysis of the skeletal striated muscles, including the diaphragm and the rest of the respiratory muscles; this would eventually cause death by asphyxiation.

Potassium chloride: stops the heart, and thus causes death by cardiac arrest.