Neomycin is overwhelmingly used as a topical preparation. It can also be given orally, where it is usually combined with other antibiotics. Neomycin is not absorbed from the gastrointestinal tract, and has been used as a preventative measure for hepatic encephalopathy and hypercholesterolemia. By killing bacteria in the intestinal tract, it keeps ammonia levels low and prevents hepatic encephalopathy, especially prior to GI surgery. It has also been used to treat small intestinal bacterial overgrowth. It is not given intravenously, as neomycin is extremely nephrotoxic (causes kidney damage), especially compared to other aminoglycosides. The exception to this, is when it is included in some vaccines as a preservative, but in very small quantities -typically 0.025 mg per dose.
Similar to other aminoglycosides, neomycin has excellent activity against Gram negative bacteria, and has partial activity against Gram positive bacteria. It is relatively toxic to humans, and some people have allergic reactions to it. See: Hypersensitivity.
Neomycin was discovered in 1949 by the microbiologist Selman Waksman and his student Hubert Lechevalier. It is produced naturally by the bacterium Streptomyces fradiae.