Special populations Doses and scheduling of antibiotics may need to be adjusted for the following: Infants Older people Patients with renal failure (see table Usual Doses of Commonly Prescribed Antibiotics) Patients with hepatic insufficiency (most commonly for cefoperazone, chloramphenicol, metronidazole, rifabutin, and rifampin) Obese patients Patients with cystic fibrosis Pregnancy and breastfeeding affect choice of antibiotic.