Neomycin

Neomycin is an aminoglycoside that is poorly absorbed from the gut, so oral use acts locally in the bowel. It is used for pre-operative bowel preparation and, less often now, for hepatic encephalopathy. Topical forms are used on skin, eye and ear. Parenteral use is abandoned because of toxicity.

Classification. Aminoglycoside.

Mechanism. Binds the 30S ribosomal subunit and causes misreading of mRNA; bactericidal, concentration-dependent.

Spectrum. Aerobic Gram-negative bacilli of the gut flora and Staphylococcus aureus. No activity against anaerobes, which is why it is paired with an anaerobic agent for bowel preparation.

Pharmacokinetics. Oral absorption under 3% in intact gut, but absorption rises with inflamed or ulcerated bowel and with renal impairment. Absorbed drug is cleared by the kidneys. Half-life 2 to 3 hours.

Maximum oral dose 12 g daily, for no more than 3 days in bowel preparation and no more than 6 days in hepatic encephalopathy.

Adults, oral. - Bowel preparation before colorectal surgery: neomycin 1 g with erythromycin 1 g at 1 pm, 2 pm and 11 pm on the day before surgery, together with mechanical cleansing. - Hepatic encephalopathy (alternative to rifaximin or lactulose): 1 g every 6 to 8 hours for up to 6 days. - Topical: apply to skin 1 to 3 times daily for up to 7 days; ear and eye preparations as the product label states.

Children. Bowel preparation 25 mg/kg a dose at the same three times. Hepatic encephalopathy 50 to 100 mg/kg/day in 3 to 4 divided doses (maximum 4 g daily).

Alternatives. Rifaximin and lactulose (encephalopathy); metronidazole with an oral cephalosporin or IV prophylaxis (surgery).

Contraindications. Intestinal obstruction; ulcerative colitis or other bowel inflammation with extensive mucosal damage; myasthenia gravis; known mitochondrial 12S rRNA mutation (m.1555A>G); aminoglycoside hypersensitivity; moderate to severe renal impairment for oral use beyond one day.

Adverse effects. Nausea, diarrhoea, malabsorption syndrome with prolonged use. Serious: ototoxicity and nephrotoxicity from absorbed drug (permanent deafness reported), neuromuscular blockade, Clostridioides difficile infection, superinfection, contact allergy with topical use.

Interactions. Warfarin (INR rise through gut flora reduction), digoxin (reduced absorption), methotrexate and oral penicillin V (reduced absorption), oral anticoagulants, loop diuretics, other aminoglycosides, vancomycin, ciclosporin and neuromuscular blockers.

Common prescribing errors. Prolonged oral courses in renal impairment, use in bowel obstruction or colitis, giving more than one aminoglycoside, and omitting anaerobic cover in bowel preparation.

Special groups. - Pregnancy: avoid systemic use; foetal eighth nerve damage is described with parenteral aminoglycosides.

Breastfeeding: oral use is acceptable (minimal absorption). - Older adults: higher risk of absorbed toxicity; use short courses. - Kidney impairment: avoid oral courses beyond one day; accumulation causes ototoxicity and nephrotoxicity. - Liver impairment: usual dose; this is the target group in encephalopathy, but hepatorenal injury raises the risk of absorbed drug. - Region: availability in East Africa is variable; locally prepared topical combinations are more common than oral tablets.

Administration. Give oral doses with water. For bowel preparation, time doses carefully against the operating list and combine with mechanical cleansing.

Baseline and monitoring. - Baseline: creatinine, hearing history, bowel integrity, medication review, pregnancy status. - During treatment: creatinine and hearing review if the course exceeds 3 days; INR in warfarin users.

Stop rules. Stop for new tinnitus, hearing loss, vertigo, rising creatinine, severe diarrhoea or neuromuscular weakness.

Referral and escalation. Refer for hearing loss, acute kidney injury, and encephalopathy that fails to improve.