Gentamicin Sulfate

Aminoglycoside Antibiotic Antibacterial

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog IV, IM, SC 6-9 mg/kg (as sulfate) once daily or divided q8h q24h or q8h Duration: 5-7 days; adjust based on culture and sensitivity
Notes: Once-daily dosing is preferred to reduce nephrotoxicity. Monitor renal function.
Cat IV, IM, SC 6-9 mg/kg (as sulfate) once daily or divided q8h q24h or q8h Duration: 5-7 days
Notes: Cats are more sensitive to nephrotoxicity; use with caution.
Horse IV, IM 6.6 mg/kg IV once daily or 2.2 mg/kg q8h q24h or q8h Duration: 5-7 days
Notes: Once-daily dosing is recommended. Monitor renal function.
Cattle IM, SC, Intramammary 4.4-6.6 mg/kg IM/SC once daily; Intramammary: 500 mg per infected quarter q24h Duration: 3-5 days
Notes: Not approved for use in cattle in some countries; extra-label use requires veterinary oversight. Observe withdrawal times.
Small Ruminants (Sheep, Goats) IM, SC 5-6 mg/kg once daily q24h Duration: 3-5 days
Notes: Extra-label use; monitor renal function.
Rabbit SC, IM, IV 2-4 mg/kg q8-24h q8-24h Duration: 5-7 days
Notes: Use with caution; nephrotoxicity is a concern.
Birds/Poultry IM, SC, Oral (drinking water) 5-10 mg/kg IM/SC q8-12h; Oral: 50-100 mg/L drinking water q8-12h Duration: 3-5 days
Notes: Extra-label; ensure adequate hydration.
Exotic/Other IM, SC Varies by species; e.g., reptiles: 2-4 mg/kg q24-72h Varies Duration: Varies
Notes: Consult a specialist for specific dosing.

Clinical Indications & Species Uses

General Indications
  • Treatment of serious infections caused by susceptible Gram-negative bacteria
  • Empirical therapy for sepsis (in combination with beta-lactams)
  • Topical treatment for otitis externa and ophthalmic infections
Dog (Canine)
  • Treatment of Gram-negative bacterial infections (e.g., E. coli, Klebsiella, Pseudomonas, Proteus)
  • Septicemia
  • Urinary tract infections
  • Skin and wound infections
  • Respiratory infections
  • Bone and joint infections
  • Peritonitis
  • Otitis externa (topical)
Cat (Feline)
  • Treatment of Gram-negative bacterial infections
  • Septicemia
  • Urinary tract infections
  • Skin and wound infections
  • Respiratory infections
  • Peritonitis
Horse (Equine)
  • Treatment of Gram-negative bacterial infections
  • Septicemia in foals
  • Pneumonia
  • Peritonitis
  • Endometritis
  • Wound infections
  • Osteomyelitis
Cattle (Bovine)
  • Treatment of Gram-negative bacterial infections
  • Mastitis (intramammary)
  • Metritis
  • Pneumonia
  • Septicemia
  • Foot rot
Small Ruminants (Sheep / Goat)
  • Treatment of Gram-negative bacterial infections
  • Mastitis
  • Pneumonia
  • Septicemia
Rabbit & Small Mammals
  • Treatment of Gram-negative bacterial infections (e.g., Pasteurella multocida)
  • Respiratory infections
  • Abscesses
  • Urinary tract infections
Avian & Poultry
  • Treatment of Gram-negative bacterial infections
  • Colibacillosis
  • Salmonellosis
  • Respiratory infections
Exotic & Other Species
  • Treatment of Gram-negative bacterial infections in reptiles, small mammals, and exotic pets

Pharmacology & Mechanism of Action

Drug Class: Aminoglycoside Antibiotic | Pharmacological Group: Antibacterial

Mechanism of Action: Gentamicin is a bactericidal aminoglycoside that irreversibly binds to the 30S ribosomal subunit, causing misreading of the genetic code and inhibition of protein synthesis. It also disrupts the bacterial cell membrane, leading to rapid cell death. Its activity is concentration-dependent and primarily effective against aerobic Gram-negative bacteria.

Pharmacodynamics: Gentamicin exhibits rapid, concentration-dependent killing of susceptible bacteria. It has a post-antibiotic effect (PAE) that persists even after drug levels fall below the MIC. It is most effective in an alkaline environment and is synergistic with beta-lactam antibiotics (e.g., penicillins) against many Gram-positive and Gram-negative organisms. Resistance can occur via plasmid-mediated enzymes that modify the drug, reducing its affinity for the ribosome.

⚑ Pharmacokinetics Summary

Absorption: Gentamicin is poorly absorbed from the gastrointestinal tract; oral administration is used for local GI effects. After parenteral administration (IM, SC), absorption is rapid and complete. Peak serum concentrations occur within 30-90 minutes after IM or SC injection.
Distribution: Gentamicin is hydrophilic and distributes primarily into extracellular fluid. It penetrates poorly into the CSF, eye, and prostate. It accumulates in the renal cortex and inner ear, which is the basis for its nephrotoxicity and ototoxicity. It crosses the placenta and is distributed into milk.
Metabolism: Gentamicin undergoes minimal metabolism; it is excreted largely unchanged by the kidneys.
Excretion: Gentamicin is eliminated primarily by glomerular filtration. In animals with normal renal function, the elimination half-life is approximately 1-3 hours in dogs and cats, 2-3 hours in horses, and 1-2 hours in cattle. Excretion is prolonged in neonates and animals with renal impairment.
Half-Life: Dogs: 1-2 hours; Cats: 1-2 hours; Horses: 1.5-3 hours; Cattle: 1-2 hours; Rabbits: 1-2 hours
Bioavailability: Oral: <10% (not systemically absorbed); IM/SC: ~100%
Protein Binding: Low protein binding (approximately 10-20%)

Available Formulations & Strengths

Injectable Solution 100 mg/mL (as sulfate) (IV, IM, SC)
Ophthalmic Solution 0.3% (3 mg/mL) (Ophthalmic)
Ophthalmic Ointment 0.3% (Ophthalmic)
Topical Cream/Ointment 0.1% (Topical)
Intramammary Infusion 500 mg per syringe (Intramammary)
Oral Solution (for poultry) 50 mg/mL (Oral)

Contraindications & Clinical Warnings

Contraindications:
  • Known hypersensitivity to gentamicin or other aminoglycosides
  • Pre-existing renal disease or severe renal impairment
  • Dehydration or hypovolemia (increases risk of nephrotoxicity)
  • Concurrent use of other nephrotoxic drugs (e.g., NSAIDs, amphotericin B, cyclosporine)
  • Pregnancy (especially in mares and cows) unless benefits outweigh risks
  • Neuromuscular disorders (e.g., myasthenia gravis) due to potential neuromuscular blockade
Warnings & Clinical Precautions:
  • Monitor renal function (serum creatinine, BUN, urinalysis) before and during therapy, especially in neonates, geriatric animals, and those with pre-existing renal disease.
  • Ensure adequate hydration during treatment.
  • Use with caution in animals with hepatic disease (though not primarily hepatotoxic).
  • Avoid concurrent use with other aminoglycosides or nephrotoxic drugs.
  • In food animals, observe withdrawal times to prevent violative residues.
  • Do not use in horses intended for human consumption (prohibited in the US).
  • Topical use on large wounds may result in systemic absorption.
  • Use in animals with neuromuscular disorders may exacerbate muscle weakness.

Adverse Effects & Reactions

Common:

  • Nephrotoxicity (increased BUN, creatinine, proteinuria)
  • Ototoxicity (vestibular and cochlear)
  • Gastrointestinal upset (nausea, vomiting, diarrhea) after oral administration
  • Injection site pain or swelling

Serious / Severe:

  • Acute renal failure
  • Irreversible hearing loss
  • Neuromuscular blockade leading to respiratory depression
  • Anaphylaxis (rare)

Rare:

  • Hypersensitivity reactions (skin rash, fever)
  • Eosinophilia
  • Hepatotoxicity (rare)
  • Electrolyte disturbances (hypokalemia, hypocalcemia)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Penicillins (e.g., ampicillin, penicillin G) Synergistic antibacterial effect; however, do not mix in same syringe or solution as they may inactivate each other. Moderate
Cephalosporins Additive nephrotoxicity; increased risk of renal damage. High
NSAIDs (e.g., flunixin, carprofen) Increased risk of nephrotoxicity due to reduced renal blood flow. High
Loop diuretics (e.g., furosemide) Increased risk of ototoxicity and nephrotoxicity. High
Neuromuscular blocking agents (e.g., atracurium, succinylcholine) Enhanced neuromuscular blockade, leading to respiratory paralysis. High
Amphotericin B Severe nephrotoxicity; avoid concurrent use. High
Methoxyflurane (anesthetic) Increased risk of nephrotoxicity. Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Acute renal failure (oliguria, anuria, elevated BUN/creatinine)
  • Ototoxicity (head tilt, ataxia, hearing loss)
  • Neuromuscular blockade (muscle weakness, respiratory depression)
  • Gastrointestinal signs (vomiting, diarrhea)

Emergency Treatment Protocol: Discontinue drug immediately. Provide supportive care: IV fluids to maintain diuresis (e.g., 0.9% saline at 1.5-2 times maintenance) to enhance renal excretion. Monitor renal function and electrolytes. In severe cases, hemodialysis or peritoneal dialysis may be considered. Calcium gluconate or neostigmine may partially reverse neuromuscular blockade. Symptomatic and supportive treatment is essential.

Food Animal Withdrawal Times

πŸ₯© Meat: 18 daysπŸ₯› Milk: 72 days

Withdrawal times vary by country and formulation. In the US, gentamicin is not approved for use in food animals; extra-label use requires extended withdrawal times (e.g., 18 days for meat, 72 hours for milk in cattle). For poultry, withdrawal times are not established; avoid use in layers producing eggs for human consumption. Always consult regulatory guidelines.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25Β°C (68-77Β°F); avoid freezing.

Handling & Special Conditions: Protect from freezing. Keep container tightly closed. For ophthalmic solutions, store upright and discard after 30 days of opening.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for human use; veterinary formulations are available but not all are FDA-approved for animals. Some products are approved for dogs, cats, and horses, but not for food animals.

Extra-Label (Off-Label) Use: In the US, gentamicin is not approved for use in food animals; extra-label use is permitted under AMDUCA with veterinary oversight, requiring extended withdrawal times. In horses, it is not approved for use in animals intended for human consumption. In the EU, use in food animals is restricted; check local regulations.

Clinical Pearls & Practice Notes

πŸ’‘ Clinical Insights: Gentamicin is a valuable antibiotic for serious Gram-negative infections, but its use is limited by nephrotoxicity and ototoxicity. Once-daily dosing is recommended to maximize efficacy and minimize toxicity. Always confirm susceptibility via culture and sensitivity testing. Monitor renal function and hydration status. In food animals, strict adherence to withdrawal times is essential. For topical use, consider potential systemic absorption. Use in combination with beta-lactams for synergistic coverage in sepsis. Avoid concurrent use with other nephrotoxic drugs.

References & Literature

  • πŸ“š Plumb's Veterinary Drug Handbook
  • πŸ“š Papich Veterinary Pharmacology and Therapeutics
  • πŸ“š Compendium of Veterinary Products (CVP)