Polymyxin B sulfate and trimethoprim are antibiotics used to treat bacterial infections of the eye, including conjunctivitis (pinkeye) and blepharoconjunctivitis (infection of the membrane that covers the outside of the eyeball and the inside and outer parts of the eyelid). Polymyxin B works by killing the bacteria, while trimethoprim works by stopping the growth of the bacteria. This medication is available as a solution (liquid) to instill in the eye, and it is usually instilled once in the affected eye(s) every three hours (maximum of 6 doses in 24 hours) for 7 to 10 days. It is important to follow the directions on the prescription label carefully and not to use more or less of it or use it more often than prescribed by the doctor.