In Madison v. Alabama the Supreme Court held 5-3 that the Eighth Amendment prohibits a person who lacks a “rational understanding” due to mental illness for why the death penalty has been imposed to be put to death regardless of what mental illness the person is suffering from.
Vernon Madison was sentenced to death for killing a police officer in 1985. Since then he has suffered a series of strokes and has been diagnosed with vascular dementia. He claims he no longer remembers the crime for which he has been sentenced to death.
In Ford v. Wainwright (1986), the Supreme Court held that the Eighth Amendment’s ban on cruel and unusual punishments disallows executing a person who has “lost his sanity” after sentencing. The Court “clarified the scope of that category in Panetti v. Quarterman  by focusing on whether a prisoner can ‘reach a rational understanding of the reason for [his] execution.’”