Formulary
Gentamicin: Uses, Dosing, Side Effects and Indian Brand Names
Gentamicin is the most used aminoglycoside, bactericidal against aerobic Gram-negatives with concentration-dependent killing and a significant post-antibiotic effect, requiring TDM due to nephrotoxicity and ototoxicity.
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Gentamicin: Uses, Dosing, Side Effects and Indian Brand Names
Gentamicin is the most used aminoglycoside, bactericidal against aerobic Gram-negatives with concentration-dependent killing and a significant post-antibiotic effect, requiring TDM due to nephrotoxicity and ototoxicity.
NEET PG High-Yield: 30S ribosomal subunit -- causes mRNA misreading (not just inhibition). Concentration-dependent killing + post-antibiotic effect once-daily dosing rationale. NOT effective against anaerobes (requires O2-dependent transport). Nephrotoxicity (reversible) and ototoxicity (irreversible). Gentamicin = vestibulotoxic; amikacin = cochleotoxic. Synergy with cell wall agents for enterococcal endocarditis. Neuromuscular blockade risk in myasthenia gravis. Category D in pregnancy. TDM essential.
Clinical overview
Gentamicin is the most widely used aminoglycoside antibiotic, active primarily against aerobic Gram-negative bacilli including Pseudomonas aeruginosa, Enterobacteriaceae (E. coli, Klebsiella, Proteus, Serratia), and in synergistic combination with cell wall-active agents (penicillin, ampicillin, vancomycin) for Gram-positive endocarditis. Aminoglycosides are NOT effective against anaerobes (they require oxygen-dependent active transport to enter the bacterial cell) -- this is a crucial distinction from their ribosomal-level mechanism. Gentamicin exhibits concentration-dependent killing and a prolonged post-antibiotic effect (PAE) of 2-8 hours, providing the pharmacokinetic basis for extended-interval (once-daily) dosing, which is now standard practice because it maximises the peak-to-MIC ratio and reduces toxicity. The two signature toxicities are nephrotoxicity (proximal tubular damage, usually reversible) and ototoxicity (damage to cochlear and vestibular hair cells, often irreversible). Gentamicin is particularly vestibulotoxic (causing vertigo and ataxia), while amikacin is more cochleotoxic (causing hearing loss). Both toxicities are potentiated by concurrent use of other nephrotoxic/ototoxic drugs and by duration of therapy beyond 7-10 days. Therapeutic drug monitoring (TDM) is essential -- target peak 8-10 mcg/mL and trough <2 mcg/mL for traditional dosing; for extended-interval dosing, a single level at 6-14 hours is plotted on a nomogram. Gentamicin is not absorbed orally and must be given parenterally (IV or IM) for systemic infections.
Pharmacological class
Gentamicin belongs to the Aminoglycoside Antibiotics class. Binds irreversibly to the 30S ribosomal subunit (specifically the 16S rRNA at the decoding site), causing misreading of mRNA codons and incorporation of incorrect amino acids into proteins. This produces non-functional or toxic proteins that insert into the bacterial cell membrane, increasing permeability and leading to cell death. Aminoglycosides are bactericidal and exhibit concentration-dependent killing with a significant post-antibiotic effect (PAE), which allows once-daily dosing.
Indian brand names and formulations
Available as: Garamycin (Glenmark), Genticyn (Abbott), Gentamicin IP (various generic), Gentalab (Laborate).
Injection: 10 mg/mL, 40 mg/mL (2 mL ampoules: 20 mg or 80 mg). Eye drops: 0.3%. Eye ointment: 0.3%. Cream: 0.1%. Ear drops: 0.3% (with betamethasone in FDC). Impregnated PMMA beads (for osteomyelitis, limited availability in India).
Regulatory status
Gentamicin is classified under Schedule H in India under the Drugs and Cosmetics Act, 1940. Prescription-only. Injectable only for systemic use. Topical formulations (eye/ear drops, cream) more widely dispensed. Reserve antibiotic per AMR stewardship protocols.
Indications
- Serious Gram-negative infections (septicaemia, hospital-acquired pneumonia, complicated UTI)
- Synergistic combination for infective endocarditis (with penicillin for streptococci, ampicillin for enterococci, vancomycin for MRSA)
- Intra-abdominal infections (with metronidazole or clindamycin for anaerobic coverage)
- Febrile neutropenia (empiric, with anti-pseudomonal beta-lactam)
- Pyelonephritis (severe, IV)
- Plague (Yersinia pestis) and tularemia (Francisella tularensis)
Dosing
Traditional dosing: 1-2.5 mg/kg every 8 hours. Extended-interval (once-daily): 5-7 mg/kg once daily (preferred -- better efficacy, less toxicity). Endocarditis synergy: 1 mg/kg every 8 hours (lower doses for synergy). Duration: usually 7-10 days maximum. Renal impairment: adjust using TDM -- extend interval or reduce dose. Obese patients: use adjusted body weight.
Contraindications
- Pre-existing significant hearing loss or vestibular dysfunction
- Myasthenia gravis (neuromuscular blockade risk)
- Concurrent use with other ototoxic or nephrotoxic agents (relative)
- Pregnancy (Category D -- risk of foetal ototoxicity, particularly cochlear damage)
Adverse effects
- Nephrotoxicity (proximal tubular damage; usually reversible; manifests as rising creatinine, proteinuria, granular casts; risk increases with duration >7 days, dehydration, and concurrent nephrotoxins)
- Ototoxicity -- vestibular (vertigo, nystagmus, ataxia -- gentamicin is more vestibulotoxic) and cochlear (hearing loss, tinnitus -- amikacin is more cochleotoxic); may be irreversible
- Neuromuscular blockade (rare; risk in myasthenia gravis, with concurrent neuromuscular blockers, or with rapid IV bolus; reversed by calcium gluconate)
- Allergic reactions (rare)
Drug interactions
- Loop diuretics (furosemide, ethacrynic acid): additive ototoxicity and nephrotoxicity; avoid concurrent use if possible
- Vancomycin: additive nephrotoxicity; monitor creatinine and drug levels closely
- Non-depolarising neuromuscular blockers (vecuronium, pancuronium): potentiated neuromuscular blockade; prolonged paralysis post-operatively
- Amphotericin B: additive nephrotoxicity
- Penicillins: synergistic bactericidal activity (desired effect in endocarditis); but physically incompatible in IV lines -- do NOT mix in same infusion
Pregnancy and lactation
Category D. Aminoglycosides cross the placenta and can cause irreversible bilateral congenital deafness in the foetus (8th cranial nerve damage). Streptomycin carries the highest foetal ototoxicity risk. Use in pregnancy only for life-threatening infections with no alternatives.
Exam-style clinical scenario
A patient with prosthetic valve endocarditis caused by Enterococcus faecalis requires combination antibiotic therapy. Why is gentamicin added to ampicillin? Gentamicin alone cannot kill enterococci (inherently resistant to aminoglycosides at standard concentrations). However, ampicillin damages the cell wall, allowing gentamicin to penetrate and bind the ribosome -- producing synergistic bactericidal activity. This synergy is essential for cure of enterococcal endocarditis.
Cost in India
Rs 5-15 per 80 mg/2 mL ampoule (IV/IM). Eye drops (0.3%): Rs 15-30. Cream: Rs 20-40 per tube.
Clinical governance
Author: MedNext Editorial Team. Clinical reviewer: Awaiting clinical review. Jurisdiction: India (Drugs and Cosmetics Act, 1940). Sources: CIMS India, Indian Pharmacopoeia. Publication state: Awaiting clinical review. Correction: Report errors at support@mednext.academy.
Frequently Asked Questions
Why does once-daily dosing reduce aminoglycoside toxicity?
Nephrotoxicity correlates with drug accumulation in proximal tubular cells, which is a saturable process. Once-daily dosing produces a high peak (maximising bactericidal effect via concentration-dependent killing) followed by a prolonged drug-free interval that allows tubular cells to recover and reduces total drug accumulation. The post-antibiotic effect (2-8 hours) maintains bactericidal activity even when serum levels fall below MIC.
Why are aminoglycosides ineffective against anaerobes?
Aminoglycosides enter bacterial cells through an oxygen-dependent active transport mechanism linked to the electron transport chain. Anaerobic bacteria lack aerobic electron transport and therefore cannot actively transport aminoglycosides into the cell. This is why gentamicin and all aminoglycosides are clinically inactive against anaerobic organisms, regardless of their ribosomal mechanism.
What is the enterococcal aminoglycoside synergy concept for NEET PG?
Enterococci are intrinsically resistant to aminoglycosides at achievable serum concentrations (MIC > 500 mcg/mL). However, cell wall-active agents (ampicillin, penicillin, vancomycin) disrupt the cell wall, allowing aminoglycosides to enter the cell and reach the ribosome. This combination produces synergistic bactericidal killing. HOWEVER, if the enterococcus has high-level aminoglycoside resistance (HLAR), synergy is lost. HLAR screening is essential before starting this combination.
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