Mechanism of Action

Aminoglycosides irreversibly bind the 30S ribosomal subunit, causing misreading of mRNA and aberrant proteins that damage the bacterial cell membrane. They are bactericidal (kill, not just inhibit) and concentration-dependent: a higher peak above the MIC means more killing. This is why extended-interval (once-daily) dosing is preferred — one large dose gives a high peak, then a drug-free interval lowers toxicity. They have poor oral absorption, so systemic use must be IV or IM; oral neomycin is the exception (stays in the gut for bowel sterilization or hepatic encephalopathy).

Common Medications

gentamicinPrototype

tobramycin

amikacin

broader gram-negative coverage; higher target levels

neomycin

oral — bowel sterilization / hepatic encephalopathy

streptomycin

Indications

serious gram-negative infections Hallmark

Pseudomonas aeruginosa

gram-negative sepsis

complicated urinary tract infection

gram-negative pneumonia

endocarditis synergy

combined with a beta-lactam or vancomycin

Contraindications & Interactions

Contraindications

pre-existing renal impairment

renally cleared — reduced clearance drives toxicity

IV push administration

never push — must infuse over 30–60 min

Interactions

loop diuretics

furosemide — additive nephro- and ototoxicity

vancomycin

additive nephro/ototoxicity

amphotericin B

additive nephrotoxicity

NSAIDs

additive nephrotoxicity

IV contrast dye

additive nephrotoxicity

Administration & Monitoring

Peak vs trough serum targets by agent

Gentamicin peak5–10 mcg/mL

Gentamicin trough0–2 mcg/mL

Tobramycin peak5–10 mcg/mL

Tobramycin trough0–2 mcg/mL

Amikacin peak20–30 mcg/mL

Amikacin trough0–8 mcg/mL

030 mcg/mL

Patient Teaching

report ringing in the ears

tinnitus — an early ototoxicity sign

report any hearing change

report dizziness or loss of balance

vestibular toxicity

drink plenty of fluids

unless contraindicated

blood will be drawn around your doses

to time peak and trough levels

hearing loss may be permanent

so early reporting matters

Report Nowescalate immediately

ototoxicity HallmarkBlack Box

FDA boxed warning (parenteral aminoglycosides); cochlear (hearing loss, tinnitus) + vestibular (vertigo, ataxia) CN VIII damage — frequently IRREVERSIBLE; stop the drug at first sign

tinnitus

early ototoxicity sign — report immediately and hold the drug

high-frequency hearing loss

starts before the client notices

vertigo

vestibular branch of CN VIII

ataxia

balance changes

nephrotoxicityrising creatinine

also a boxed warning; usually reversible if caught early; rising creatinine/BUN signals accumulation

elevated trough levelgentamicin/tobramycin > 2 mcg/mL

strongest predictor of toxicity; hold dose and notify provider

oliguriaurine output < 30 mL/hr

impaired renal clearance accelerates accumulation

Clinical Pearl

Trough predicts toxicity, peak predicts efficacy — draw the trough just before the dose and the peak 30 minutes after. If the trough won't come down, the kidneys can't keep up: hold the dose and call the provider. And the kidneys may recover, but the hearing often won't — stop the drug at the first tinnitus.