What is the ruling on performing a fecal microbiota transplant from a donor to a patient suffering from a deficiency in beneficial bacteria, for both the doctor and the patient? And does the ruling differ between a Muslim and a disbeliever, given the existence of less effective alternatives?

The summary of the answer addresses three points: First, human feces are impure by consensus. Second, treatment with impure substances is a matter of scholarly disagreement, with the more apparent view being its permissibility out of necessity when no pure treatment is available, and on the condition that a just Muslim physician is informed. Third, using impure substances via injection or topical application is less severe than ingesting or drinking them. In cases where treatment with feces is necessary, it should be used via injection and not orally or nasally.