Fidaxomicin, sold under the brand name Dificid (by Merck) among others, is the first member of a class of narrow spectrum macrocyclic antibiotic drugs called tiacumicins.

[4][5] Fidaxomicin is minimally absorbed into the bloodstream when taken orally, is bactericidal, and selectively eradicates pathogenic Clostridioides difficile with relatively little disruption to the multiple species of bacteria that make up the normal, healthy intestinal microbiota.

TiaB, which codes for another type I PKS, forms an homoorsellinic acid moiety from propionyl-CoA and three malonyl-CoA elongating units that is coupled to rhamnose at the OH-C(4’) position by the TiaF, a ketoacyl ACP synthase.

[11] Good results were reported by the company in 2009, from a North American Phase III clinical trial comparing it with oral vancomycin for the treatment of Clostridioides difficile infection.

[15] Fidaxomicin was shown to be as good as the standard-of-care, vancomycin, for treating Clostridioides difficile infection in a Phase III clinical trial published in February 2011.

[21] The most common side effects reported in adults with the use of fidaxomicin include nausea, abdominal pain, vomiting, anemia, neutropenia, and gastrointestinal hemorrhage.

[21] In children the most common side effects include fever, vomiting, diarrhea, constipation, abdominal pain, rash, and increased aminotransferases.