Fidaxomicin is a novel macrocyclic antibiotic recently approved by the US Food and Drug Administration for the treatment of Clostridium difficile-associated diarrhea in adults. We reviewed safety data from nonclinical studies and clinical trials (phases 1, 2A, and 3) with fidaxomicin. In nonclinical studies, fidaxomicin was administered orally at approximately 1 g/kg/d to dogs for up to 3 months with no significant target-organ toxicities observed. A total of 728 adults have received oral fidaxomicin in clinical trials to date: 116 healthy volunteers and 612 patients with C. difficile infection. In phase 3 clinical trials, fidaxomicin was well tolerated, with a safety profile comparable with oral vancomycin. There were no differences in the incidence of death or serious adverse events between the 2 drugs. Fidaxomicin appears to be well tolerated. Continued monitoring of adverse events in the postmarketing setting will provide additional information about the full safety profile of fidaxomicin.