Gentamicin !

The workhorse injectable aminoglycoside for serious gram-negative infections and, with penicillin, for some endocarditis; harmful to kidney and ear.

Updated 2026-11-01

Class: Aminoglycosides
Target: Decoding (A) site of the bacterial 16S rRNA, 30S ribosomal subunit
ATC code: J01GB03
WHO Model List of Essential Medicines

Decoding (A) site of the bacterial 16S rRNA, 30S ribosomal subunitGentamicin

The figure shows gentamicin C1a (one component of gentamicin) bound to a short RNA that reproduces the A site of bacterial 16S rRNA, not a whole ribosome.

Target in 3D: Decoding (A) site of the bacterial 16S rRNA, 30S ribosomal subunit. PDB structure 2ET3 (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Gentamicin enters gram-negative bacteria through porins and an oxygen-dependent transport, then binds the decoding site of the 30S subunit. Ribosomes stall at the start codon or read the code wrongly, so the cell makes faulty proteins and dies. It is bactericidal, works best at high peak levels, and acts in synergy with penicillins against enterococci.

Crosses porins and the oxygen-dependent transport
Binds the 16S rRNA A site of the 30S subunit
mRNA is misread and translation stops early
Faulty proteins kill the bacterium

Pharmacokinetics

Absorption: Given intravenously or intramuscularly; aminoglycosides are not used by mouth for systemic infections.
Distribution: Stays in the extracellular fluid with little protein binding; concentrates in the renal cortex and the inner-ear fluids.
Metabolism: Little or no metabolism.
Elimination: Unchanged in the urine by glomerular filtration; the half-life tracks creatinine clearance.

Indications

Serious infections by aerobic gram-negative bacilli, including Pseudomonas aeruginosa, often started before culture results. Sepsis and septic shock →
Suspected neonatal sepsis, together with a penicillin-type drug. Neonatal sepsis →
Enterococcal or streptococcal endocarditis, in synergy with penicillin, ampicillin or vancomycin. Infective endocarditis →
Complicated urinary tract infections. Acute pyelonephritis →

Adverse effects

Common

Rising creatinine, casts or protein in the urine
Dizziness, tinnitus, a feeling of fullness in the ears
Low magnesium, calcium and potassium
Rash and other hypersensitivity reactions

Contraindications

Hypersensitivity to gentamicin
History of hypersensitivity or serious toxicity with another aminoglycoside (cross-sensitivity)

Key interactions

Loop diuretics (furosemide, ethacrynic acid): Added ototoxicity; the combination is avoided.
Vancomycin, cisplatin, polymyxin B, colistin, other aminoglycosides: Added kidney and nerve toxicity; avoided at the same time or one after another.
Cephalosporins: Reports of more nephrotoxicity.
Neuromuscular blockers and anaesthetics: Deeper and longer blockade, risk of apnea.
Penicillins mixed in the same solution: Inactivate the aminoglycoside in vitro: they are not mixed.

What is monitored

Peak and trough blood levels as the protocol sets
Creatinine, urine output and urinalysis
Hearing and balance (audiometry in long courses)
Magnesium, calcium and potassium

Administration and safety

Intravenous infusion or intramuscular injection; not given as a fast IV push.
Not mixed in the same bag or line with penicillins or cephalosporins: they inactivate each other.
Level samples are drawn at the exact times the protocol sets after the dose; wrong timing gives misleading results.
Absorbed from body cavities and wounds after irrigation, with the same toxicity as injection.

Nursing care

Before the first dose: weight, creatinine, hearing problems, myasthenia gravis and pregnancy.
Keep the patient well hydrated and record intake and urine output.
Ask daily about tinnitus, ear fullness, dizziness or unsteadiness.
After surgery or with neuromuscular blockers, watch breathing and muscle strength.
Teaching: report hearing change, ringing, unsteadiness, numbness or less urine.

Pregnancy and breastfeeding

Aminoglycosides cross the placenta and can harm the fetus; congenital deafness has been reported with streptomycin. Gentamicin is used in pregnancy only when the benefit justifies it.

Test yourself

Think of the answer, then tap the question to see it.

A patient on gentamicin and furosemide reports ringing in the ears. Why is the combination risky?

Both drugs are ototoxic, and IV loop diuretics can also raise aminoglycoside levels in tissue: hearing damage adds up and may be permanent.

Why is gentamicin combined with ampicillin for enterococcal endocarditis?

The cell-wall drug opens the way for the aminoglycoside to enter, and together they kill enterococci that neither drug kills well alone (synergy).

After surgery, a patient who received gentamicin irrigation breathes poorly and is weak. What is the likely cause?

Aminoglycoside neuromuscular blockade, increased by anaesthetics and neuromuscular blockers; it can cause apnea.

Why is gentamicin not mixed with penicillin in the same infusion bag?

The β-lactam inactivates the aminoglycoside in the solution, so the patient would receive less active drug.

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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 - GENTAMICIN SULFATE injection, solution (DailyMed (U.S. National Library of Medicine), 2026); Gentamicin Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 59 (McGraw Hill, 2023); RCSB PDB - 2ET3: Complex Between Gentamicin C1A and the 16S-RRNA A-Site (RCSB Protein Data Bank, 2026).

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Representative model, for educational purposes only.