Neomycin is an antibiotic, which means that it can kill some species of bacteria. When made into an ointment form, neomycin products are typically designed to help cure eye or skin infections. Some products contain neomycin along with other antibiotics or steroids. Possible side effects from using neomycin ointment include irritation and rashes; hearing can also be affected.
When bacteria infect humans, they need to produce certain molecules to help keep the cell healthy and thriving. As part of the aminoglycoside group of antibiotics, neomycin interferes with the normal workings of the bacterial cell, and prevents it from making enough normal molecules to stay alive and healthy. So when neomycin ointment is applied to the eyes or to the skin, the bacterial cells are exposed to it and the drug kills them off over time.
Commonly, antibiotics need to be taken over a period of time in order to ensure that all the infecting bacteria are killed off. Neomycin ointment is no different, and a patient generally needs to apply the ointment several times a day for as many days as specified on the label. Sometimes, neomycin ointment can be used on skin scratches to reduce the chance of infections, but as the drug can cause potentially serious side effects once absorbed into the body through broken skin, this usage may be more risky than applying it to areas that are already infected.
The most common side effects of neomycin ointment occur on the skin or eyes, as these are the most common areas the drug is applied. It can cause an itchy feeling on the skin, or a rash. Sometimes the person using it can experience burning where it is applied, and if used in the eyes, vision can be temporarily blurry. More severe effects include a deterioration in hearing ability, or an abnormally low urine output.
Risk of serious side effects increases if neomycin ointment is used at the same time as some other medications. These are primarily other antibiotics such as vancomycin, cephalosporin or polymixin, but diuretics like furosemide can also pose problems. Some medications are safe to take in conjunction with neomycin, and these are sometimes found in similar products with the drug. Bacitracin is one example, and steroid drugs like hydrocortisone, methylpredisolone or dexamethasone are also common additions to neomycin products. Combination products, however, carry different side effects due to the additional ingredients.