Neomycin is an antibiotic used to prevent and treat bacterial infections in several distinct ways: as a topical ointment for minor skin wounds, as an oral medication to reduce ammonia in liver disease, and as part of bowel preparation before colorectal surgery. You’ve likely encountered it without knowing the name, since it’s one of the three active ingredients in Neosporin and most store-brand “triple antibiotic” ointments.
How Neomycin Works
Neomycin kills bacteria by interfering with their ability to build proteins. It binds to part of the bacterial ribosome (the cell’s protein-making machinery) and causes it to misread genetic instructions. The result is that bacteria produce defective, nonfunctional proteins and essentially self-destruct. It works primarily against gram-negative bacteria, with partial activity against gram-positive species. This broad coverage is what makes it useful across such different applications, from skin ointments to gut decontamination.
Topical Use for Skin Infections
The most common way people encounter neomycin is in over-the-counter antibiotic ointments applied to minor cuts, scrapes, and burns. In these products, neomycin is typically combined with two other antibiotics (bacitracin and polymyxin B) to cover a wider range of bacteria. Brand names include Neosporin Original, along with dozens of generic and store-brand versions labeled “triple antibiotic ointment.” Some formulations also include a mild numbing agent for pain relief.
For minor skin infections, the ointment is applied one to three times a day. It’s meant for shallow wounds only. Deep puncture wounds, serious burns, and large raw areas of skin need professional medical attention rather than an over-the-counter ointment, because the antibiotic can’t penetrate deeply enough to prevent infection in those situations and the wound itself may need more advanced care.
Treating Liver-Related Brain Fog
Oral neomycin plays a very different role in managing hepatic encephalopathy, a condition where a damaged liver can no longer filter ammonia from the blood. As ammonia builds up, it reaches the brain and causes confusion, personality changes, drowsiness, and in severe cases, coma. This is most common in people with advanced cirrhosis.
When taken by mouth, neomycin stays mostly in the gut rather than being absorbed into the bloodstream. There, it reduces ammonia production by inhibiting an enzyme called glutaminase, which normally converts one amino acid into another while releasing ammonia as a byproduct. By suppressing this process, neomycin helps lower the ammonia levels that drive neurological symptoms. The typical oral dose ranges from 1 to 4 grams daily, split into multiple doses. It’s generally used as a second-line option when the primary treatment (a synthetic sugar called lactulose) isn’t enough on its own.
Bowel Preparation Before Surgery
Surgeons performing elective colorectal procedures often prescribe oral neomycin the day before surgery as part of a bowel preparation regimen. The goal is to reduce the bacterial population in the intestines, lowering the risk of surgical site infections when the colon is opened during the operation.
Neomycin is paired with a second oral antibiotic, typically erythromycin or metronidazole, to broaden coverage against different bacterial types. Both the American College of Surgeons and the American Society of Colon and Rectal Surgeons recommend this combination before all major elective colorectal resections. The antibiotics are taken the day before surgery, usually alongside a mechanical bowel prep that clears the intestines.
Ear and Eye Drops
Neomycin also appears in prescription ear and eye drops, usually combined with polymyxin B and a steroid like hydrocortisone. These formulations treat bacterial infections of the outer ear (swimmer’s ear) or certain eye infections while the steroid component reduces inflammation and swelling. Unlike the over-the-counter skin ointments, these combination drops require a prescription.
Side Effects and Risks
Topical neomycin is generally well tolerated, but it’s one of the more common causes of allergic contact dermatitis among antibiotic ingredients. In one study of over 1,100 patients tested for contact allergies, 6% had a positive reaction to neomycin. If you notice increasing redness, itching, or a rash that seems worse than the original wound after applying a triple antibiotic ointment, a neomycin allergy is a likely explanation. Switching to a neomycin-free alternative (plain bacitracin or petroleum jelly) usually resolves it.
The more serious risks come with oral or systemic use. Neomycin can damage the kidneys and the inner ear, potentially causing permanent hearing loss even at normal doses. Warning signs of kidney problems include changes in how much you urinate, blood in the urine, or sudden unexplained weight gain from fluid retention. Signs of ear damage include ringing in the ears or any noticeable change in hearing. These risks increase in older adults, infants, people who are dehydrated, and anyone with preexisting kidney or hearing problems. Hearing loss can develop even after the medication has been stopped, which is why oral neomycin tends to be reserved for specific situations rather than used as a general-purpose antibiotic.
Why Neomycin Isn’t Used Systemically
Despite being effective against many bacteria, neomycin is almost never given by injection or used to treat infections throughout the body. Its potential for kidney and hearing damage is simply too high compared to safer alternatives. This is why its medical uses are limited to situations where it either stays on the skin surface, remains in the gut without significant absorption, or is applied in small quantities to the ear or eye. Within those boundaries, it remains a widely used and effective antibiotic decades after its introduction.