Neomycin, also known as framycetin, is an aminoglycoside antibiotic that displays bactericidal activity against Gram-negative aerobic bacilli and some anaerobic bacilli where resistance has not yet arisen. It is generally not effective against Gram-positive bacilli and anaerobic Gram-negative bacilli. Neomycin comes in oral and topical formulations, including creams, ointments, and eyedrops. Neomycin belongs to the aminoglycoside class of antibiotics that contain two or more amino sugars connected by glycosidic bonds.

Neomycin was discovered in 1949 by microbiologist Selman Waksman and his student Hubert Lechevalier at Rutgers University. Neomycin received approval for medical use in 1952. Rutgers University was granted the patent for neomycin in 1957.

Neomycin was discovered in 1949 by the microbiologist Selman Waksman and his student Hubert Lechevalier at Rutgers University. It is produced naturally by the bacterium Streptomyces fradiae. Synthesis requires specific nutrient conditions in either stationary or submerged aerobic conditions. The compound is then isolated and purified from the bacterium.

Neomycin is typically applied as a topical preparation, such as Neosporin (neomycin/polymyxin B/bacitracin). The antibiotic can also be administered orally, in which case it is usually combined with other antibiotics. Neomycin is not absorbed from the gastrointestinal tract and has been used as a preventive measure for hepatic encephalopathy and hypercholesterolemia. By killing bacteria in the intestinal tract, Neomycin keeps ammonia levels low and prevents hepatic encephalopathy. Due to its poor GI tract absorption, orally administered neomycin has also been used to reduce the risk of post operative infection following gastrointestinal surgery.

Waksman and Lechevalier originally noted that neomycin was active against streptomycin-resistant bacteria as well as Mycobacterium tuberculosis, the causative agent for tuberculosis. Neomycin has also been used to treat small intestinal bacterial overgrowth. Neomycin is not administered via injection, as it is extremely nephrotoxic (damaging to kidney function) even when compared to other aminoglycosides. The exception is when neomycin is included, in small quantities, as a preservative in some vaccines – typically 25 μg per dose.

In 2023, the combination of neomycin with dexamethasone and polymyxin B was the 260th most commonly prescribed medication in the United States, with more than 1 million prescriptions.

Similar to other aminoglycosides, neomycin has excellent activity against Gram-negative bacteria and is partially effective against Gram-positive bacteria. It is relatively toxic to humans, with allergic reactions noted as a common adverse reaction (see: hypersensitivity). Physicians sometimes recommend using antibiotic ointments without neomycin, such as Polysporin. The following represents minimum inhibitory concentration (MIC) susceptibility data for a few medically significant Gram-negative bacteria.

In 2005–06, Neomycin was the fifth-most-prevalent allergen in patch test results (10.0%). It was named Allergen of the Year in 2010. Neomycin is also a known GABA gamma-Aminobutyric acid antagonist and can be responsible for seizures and psychosis. Like other aminoglycosides, neomycin has been shown to be ototoxic, causing tinnitus, hearing loss, and vestibular problems in a small number of patients. Neomycin affects the cochlea, which is found in the inner ear. Hearing loss is caused by ear hair cell death, which occurs in response to treatment with neomycin. Patients with existing tinnitus or sensorineural hearing loss are advised to speak with a healthcare practitioner about the risks and side effects prior to taking this medication.^{[citation needed]}