The Three-Drug Protocol
Lethal injection uses a specific sequence of three drugs, each serving a different purpose. The first drug is an anesthetic, usually sodium pentothal or pentobarbital, which is meant to make the person unconscious so they do not feel pain. The second drug is potassium chloride, which causes the heart to stop beating by disrupting the electrical signals that control heart function. The third drug is pancuronium bromide, a paralytic agent that stops the muscles used for breathing, including the diaphragm.
How Death Occurs
Death results from a combination of factors. Once the anesthetic takes effect, the person loses consciousness. The potassium chloride then causes cardiac arrest by flooding the bloodstream with potassium ions, which disrupts the heart's electrical activity and causes it to stop beating. Without a beating heart, blood stops circulating, and the brain and other organs are deprived of oxygen. The paralytic drug prevents the person from showing any visible signs of distress, though medical experts have debated whether the anesthetic dose is always sufficient to prevent suffering.
Medical and Ethical Concerns
There have been significant concerns about whether lethal injection is carried out humanely. Critics argue that if the anesthetic dose is inadequate, the person could be conscious and in pain while the other drugs are administered, but unable to move or cry out due to paralysis. Medical professionals are generally prohibited from participating in executions, so the injections are often administered by non-medical personnel, which raises questions about proper technique and dosing.
Legal Status
Lethal injection is the primary method of execution in the United States for capital punishment. Different states may use slightly different drug protocols. The method has been challenged in courts on grounds of cruel and unusual punishment, but it remains the most common execution method in states that practice capital punishment. 
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