1. Clinical Overview
Gentamicin is a bactericidal aminoglycoside antibiotic derived from Micromonospora purpurea. It is a broad-spectrum antibiotic primarily effective against aerobic Gram-negative bacilli, including Pseudomonas aeruginosa, and some Gram-positive organisms like Staphylococcus aureus. In the Indian context, it is a critical, cost-effective agent for severe infections, but its use is tempered by the risk of nephrotoxicity and ototoxicity, necessitating therapeutic drug monitoring (TDM) where available.
| Onset | Duration | Bioavailability |
|---|---|---|
| Rapid; peak serum concentrations are achieved within 30-60 minutes after an intramuscular (IM) injection. | Dose-dependent; post-antibiotic effect can last several hours, allowing for once-daily dosing regimens in many cases. | Poorly absorbed from the GI tract (<1% orally). IM bioavailability is ~100%. IV administration provides complete bioavailability. |
2. Mechanism of Action
Gentamicin binds irreversibly to the 30S ribosomal subunit of susceptible bacteria, specifically to the 16S rRNA and proteins of the A-site. This binding interferes with the initiation complex, causes misreading of the mRNA template, and inhibits translocation. The result is the incorporation of incorrect amino acids into the growing peptide chain, leading to the production of non-functional or toxic proteins and ultimately bacterial cell death.
3. Indications & Uses
- Severe infections caused by susceptible Gram-negative bacilli (e.g., Pseudomonas, E. coli, Klebsiella, Enterobacter, Serratia, Proteus)
- Complicated urinary tract infections (pyelonephritis)
- Intra-abdominal infections (peritonitis)
- Septicemia (including neonatal sepsis)
- Hospital-acquired pneumonia and ventilator-associated pneumonia (VAP)
- Skin and soft tissue infections (burns, surgical wounds)
4. Dosage & Administration
Adult Dosage: Conventional dosing: 3-5 mg/kg/day in 2-3 divided doses (e.g., 80 mg IV/IM every 8 hours). Extended-Interval (Once-Daily) Dosing: 5-7 mg/kg as a single daily dose. MUST be guided by TDM and clinical condition.
Administration: For IV use: Dilute in 50-100 mL of 0.9% Sodium Chloride or 5% Dextrose. Infuse over 30-60 minutes. NEVER give as IV bolus (risk of neuromuscular blockade). For IM use: Administer deep into a large muscle mass. Monitor peak levels 30 minutes after end of IV infusion or 60 minutes after IM injection. Monitor trough levels just before the next dose.
5. Side Effects
Common side effects may include:
- Pain at injection site (IM)
- Nausea
- Vomiting
- Mild rise in serum creatinine/BUN
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Other Nephrotoxic drugs (Vancomycin, Amphotericin B, Cisplatin, NSAIDs, Loop diuretics) | Additive or synergistic nephrotoxicity | Major |
| Other Ototoxic drugs (Furosemide, Ethacrynic acid, Cisplatin) | Additive or synergistic ototoxicity | Major |
| Neuromuscular blocking agents (Succinylcholine, Tubocurarine) | Enhanced and prolonged neuromuscular blockade, leading to respiratory depression/apnea | Major |
| Penicillins (e.g., Carbenicillin, Ticarcillin) when mixed in vitro | Physical inactivation of gentamicin; administer separately. | Moderate |
| Indomethacin in neonates | May increase gentamicin levels and toxicity risk | Moderate |
7. Patient Counselling
- DO complete the full course of therapy as prescribed.
- DO inform all your doctors about your gentamicin use.
- DO maintain adequate fluid intake unless contraindicated.
- DO report any ringing in ears, dizziness, hearing difficulty, or changes in urine output immediately.
- DONT skip doses or double up on missed doses.
- DONT take any other medicines (including OTC painkillers like ibuprofen) without consulting your doctor.
8. Toxicology & Storage
Overdose: Manifestations of overdose include severe ototoxicity (hearing loss, tinnitus, vertigo), nephrotoxicity (acute tubular necrosis, oliguria), neuromuscular blockade (muscle weakness, respiratory paralysis), and encephalopathy.
Storage: Store vials/ampoules at controlled room temperature (15-25°C), protected from light. Do not freeze. After dilution for IV infusion, solutions are stable for 24 hours at room temperature or refrigerated. Discard any unused portion from a single-dose vial. Keep out of reach of children.