Pharmacology · Aminoglycosides
Neomycin !
An aminoglycoside used by mouth to suppress gut bacteria (bowel preparation, hepatic encephalopathy) and on the skin; too toxic to inject.
Updated 2026-11-01
The figure shows neomycin B bound to a short RNA that reproduces the A site of bacterial 16S rRNA, not a whole ribosome.
Mechanism of action
Neomycin binds the 30S ribosomal subunit and causes misreading, killing susceptible aerobic gut bacteria. Taken by mouth it is poorly absorbed, so it suppresses most intestinal bacteria for a short time and lowers the ammonia they produce. It is not active against the anaerobic bowel flora.
- Stays mostly in the gut after an oral dose
- Binds the 30S subunit of aerobic gut bacteria
- Ammonia-forming bacteria fall
Pharmacokinetics
- Absorption
- Poorly absorbed from the normal gut, but small amounts do enter the blood, more with inflamed or ulcerated bowel or kidney failure.
- Elimination
- The unabsorbed part leaves in the faeces; the absorbed part is cleared by the kidney.
Indications
- Adjunct in hepatic encephalopathy, by reducing ammonia-forming gut bacteria. Hepatic encephalopathy →
- Preoperative suppression of bowel flora, with another oral antibiotic. Surgical site infection →
Adverse effects
Common
- Nausea, vomiting, diarrhoea
- Malabsorption of fat, vitamin B12 and other nutrients with high doses
Contraindications
- Intestinal obstruction
- Inflammatory or ulcerative gastrointestinal disease (more absorption)
- Hypersensitivity to neomycin or serious reactions to other aminoglycosides
Key interactions
- Digoxin, penicillin V, oral vitamin B12, methotrexate, fluorouracil
- Oral neomycin reduces their absorption; digoxin levels are monitored.
- Coumarin anticoagulants (warfarin)
- Stronger anticoagulant effect by lowering vitamin K from gut bacteria.
- Loop diuretics and other nephrotoxic or ototoxic drugs
- Added toxicity; avoided together.
What is monitored
- Kidney function and urinalysis
- Hearing, especially in long courses or kidney failure
- INR in patients on warfarin
Administration and safety
- Oral tablets for bowel use; it is not given by injection.
- Topical products are only for skin, eye or ear as labelled.
Nursing care
- Check for bowel obstruction or inflammatory bowel disease before starting.
- Watch mental status and ammonia trend in hepatic encephalopathy.
- Teaching: report hearing change, ringing, less urine or muscle weakness.
Pregnancy and breastfeeding
Aminoglycosides can cause fetal harm, including congenital deafness; small amounts of oral neomycin are absorbed, so it is avoided unless clearly needed.
Test yourself
Think of the answer, then tap the question to see it.
How does oral neomycin help in hepatic encephalopathy?
It reduces ammonia-forming bacteria in the gut, lowering blood ammonia.
Why is oral neomycin contraindicated in ulcerative bowel disease?
Inflamed mucosa absorbs more drug, raising the risk of ear and kidney toxicity.
How can oral neomycin change warfarin's effect?
It reduces vitamin K made by gut bacteria, which can strengthen the anticoagulant effect.
Related drugs
Related conditions
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - NEOMYCIN SULFATE tablet (DailyMed (U.S. National Library of Medicine), 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 59 (McGraw Hill, 2023); RCSB PDB - 2ET4: Complex Between Neomycin B and the 16S-RRNA A-Site (RCSB Protein Data Bank, 2026).
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Representative model, for educational purposes only.