Typically, prohibitions are suspended in order to heal the ill. Only in rare circumstances does one leave a sick person in danger due to a prohibition that would be violated by his treatment. Even the fast of Yom Kippur is overridden if it would endanger a patient.

Abaye and Rava both said: Anything that contains an element of healing is not prohibited due to the ways of the Emorite.

When Ravin came from the Land of Israel to Babylonia, he said that Rabbi Yohanan said: One may heal himself with any item except for those prohibited due to idolatry, forbidden sexual relations, and bloodshed.

[On Yom Kippur], a sick person is fed in accordance with the advice of medical experts. If there are no experts there, one feeds him according to his own assessment, until he says that he has eaten enough.

Rabbi Yannai said: If a sick person says he needs to eat and a doctor says he does not need to eat, one heeds the sick person. What is the reason? “The heart knows its own bitterness” (Proverbs 14:10). If a doctor says that the sick person needs to eat but the sick person says he does not need to eat, one heeds the doctor. What is the reason? It is because confusion may have overcome the sick person [impairing his judgment].

In the case of one who is afflicted with overwhelming, ravenous hunger, one may feed him even forbidden foods until his eyes recover. (Shabbat 67a; Pesaĥim 25a–b; Yoma 82a, 83a; Mishna Yoma 8:6)