Polymyxin B sulfate is primarily used as an antibiotic to treat severe bacterial infections, particularly those caused by Gram-negative bacteria. It is often employed when other antibiotics are ineffective due to bacterial resistance. Common applications include treating infections of the urinary tract, bloodstream, and central nervous system, such as meningitis. It is also used topically for skin, eye, and ear infections, including conditions like conjunctivitis and otitis externa. In hospitals, it is sometimes administered intravenously or via inhalation for patients with cystic fibrosis or pneumonia caused by multidrug-resistant pathogens. Additionally, it is used in combination with other antibiotics to enhance its effectiveness and reduce the risk of resistance development.