Gentamicin Side Effects: What to Expect and When to Call Your Doctor

Gentamicin is a powerful antibiotic, and like all powerful medications, it comes with risks. Understanding what side effects are possible — and especially which ones require immediate attention — will help you use this medication safely. This guide covers what you may experience with gentamicin in its various forms: injections, eye drops, and topical cream or ointment.

The Black Box Warning: Kidney and Hearing Damage

Gentamicin injectable carries the FDA's most serious warning — a

Black Box Warning — for two serious risks:

Nephrotoxicity (kidney damage): Gentamicin can damage the kidneys, particularly at high doses or with prolonged use. The risk is higher in patients with existing kidney disease, older adults, and those who are dehydrated. Your healthcare team will monitor your kidney function with regular blood tests during IV gentamicin treatment.

Ototoxicity (hearing and balance damage): Gentamicin can damage both the auditory (hearing) and vestibular (balance) parts of the inner ear. Hearing loss from gentamicin may be permanent. Vestibular damage can cause dizziness, imbalance, and a feeling of the room spinning (vertigo). Risk is higher with higher doses, longer treatment, pre-existing kidney disease, and concurrent use of loop diuretics like furosemide.

These serious risks apply primarily to

injectable gentamicin. Ophthalmic and topical formulations have very low systemic absorption and do not carry the same risks — though patients with extensive burns may absorb topical gentamicin systemically.

Common Side Effects of Injectable Gentamicin

The following side effects may occur with gentamicin injection and should be reported to your healthcare team:

Decreased urination or changes in urine color (sign of kidney stress)

Ringing in the ears (tinnitus) — an early sign of ototoxicity

Dizziness or balance problems — vestibular toxicity

Nausea, vomiting, or loss of appetite

Fever or rash (signs of allergic or inflammatory response)

Pain at the injection or infusion site

Lab abnormalities: elevated creatinine (kidney), elevated liver enzymes, or low magnesium/potassium

When to Call Your Doctor Immediately

Stop gentamicin and contact your provider or seek emergency care if you experience:

Sudden hearing loss or significant worsening of hearing — this can be permanent if not caught early

New or worsening dizziness, vertigo, or unsteadiness — sign of vestibular damage

Significantly reduced urination or swollen legs/face — signs of acute kidney injury

Muscle weakness or difficulty breathing — neuromuscular blockade (rare but serious)

Severe allergic reaction: hives, throat tightening, difficulty breathing, or swelling of face/tongue

Side Effects of Gentamicin Eye Drops (Gentak)

Gentamicin ophthalmic has a much milder side effect profile than the injectable form because very little of the drug enters the bloodstream. Common eye-related side effects include:

Mild stinging or burning when the drops are applied

Temporary blurred vision (especially with the ointment)

Eye redness or tearing

Risk of secondary fungal infection with prolonged use — though this is uncommon with the short courses typically prescribed for eye infections

Side Effects of Topical Gentamicin Cream/Ointment

Topical gentamicin applied to intact skin has minimal systemic absorption and is generally well-tolerated. Side effects are mostly local:

Skin irritation, redness, or itching at the application site

Allergic contact dermatitis (rare but possible)

Risk of fungal overgrowth with prolonged use (apply only for the prescribed duration)

How Your Doctor Monitors for Gentamicin Side Effects

For patients receiving injectable gentamicin, your healthcare team will monitor you closely with:

Serum drug levels (TDM): Gentamicin levels are checked 6–14 hours after dosing to ensure levels are in the therapeutic range and not dangerously high.

Kidney function tests: Serum creatinine and BUN checked at baseline and during therapy, especially if treatment extends beyond 48–72 hours.

Audiological monitoring: Hearing evaluation recommended for patients on prolonged courses, those with pre-existing hearing loss, or those receiving other ototoxic drugs.

To reduce your risk, let your doctor know about all medications you are taking — especially diuretics like furosemide or other antibiotics like vancomycin. See our related guide on

Gentamicin drug interactions for a full list of medications to discuss with your doctor.