11 Del. C. § 401

(a) In any prosecution for an offense, it is an affirmative defense that, at the time of the conduct charged, as a result of mental illness or serious mental disorder, the accused lacked substantial capacity to appreciate the wrongfulness of the accused’s conduct. If the defendant prevails in establishing the affirmative defense provided in this subsection, the trier of fact shall return a verdict of “not guilty by reason of insanity.”

(b) Where the trier of fact determines that, at the time of the conduct charged, a defendant suffered from a mental illness or serious mental disorder which substantially disturbed such person’s thinking, feeling or behavior and/or that such mental illness or serious mental disorder left such person with insufficient willpower to choose whether the person would do the act or refrain from doing it, although physically capable, the trier of fact shall return a verdict of “guilty, but mentally ill.”

(c) It shall not be a defense under this section if the alleged insanity or mental illness was proximately caused by the voluntary ingestion, inhalation or injection of intoxicating liquor, any drug or other mentally debilitating substance, or any combination thereof, unless such substance was prescribed for the defendant by a licensed health-care practitioner and was used in accordance with the directions of such prescription. As used in this chapter, the terms “insanity” or “mental illness” do not include an abnormality manifested only by repeated criminal or other antisocial conduct.