Gentamicin Injection, USP may be given intramuscularly or by intravenous infusion. The patient’s pretreatment body weight should be obtained for calculation of correct dosage. The dosage of aminoglycosides in obese patients should be based on an estimate of the lean body mass. It is desirable to limit the duration of treatment with aminoglycosides to short term.
Patients with Normal Renal Function
Adults: The recommended dosage of Gentamicin for patients with serious infections and normal renal function is 3 mg/kg/day, administered in three equal doses every eight hours ().
For patients with life-threatening infections, dosages up to 5 mg/kg/day may be administered in three or four equal doses. The dosage should be reduced to 3 mg/kg/day as soon as clinically indicated ().
It is desirable to measure both peak and trough serum concentrations of Gentamicin to determine the adequacy and safety of the dosage. When such measurements are feasible, they should be carried out periodically during therapy to assure adequate but not excessive drug levels. For example, the peak concentration (at 30 to 60 minutes after intramuscular injection) is expected to be in the range of 4 to 6 mcg/mL. When monitoring peak concentrations after intramuscular or intravenous administration, dosage should be adjusted so that prolonged levels above 12 mcg/mL are avoided. When monitoring trough concentrations (just prior to the next dose), dosage should be adjusted so that levels above 2 mcg/mL are avoided. Determination of the adequacy of a serum level for a particular patient must take into consideration the susceptibility of the causative organism, the severity of the infection, and the status of the patient’s host-defense mechanisms. In patients with extensive burns, altered pharmacokinetics may result in reduced serum concentrations of aminoglycosides. In such patients treated with Gentamicin, measurement of serum concentrations is recommended as a basis for dosage adjustment.
Children: 6 to 7.5 mg/kg/day (2 to 2.5 mg/kg administered every 8 hours).
Infants and Neonates: 7.5 mg/kg/day (2.5 mg/kg administered every 8 hours).
Premature or Full-Term Neonates One Week of Age or Less: 5 mg/kg/day (2.5 mg/kg administered every 12 hours).
NOTE: For further information concerning the use of Gentamicin in infants and children, see pediatric Gentamicin injection product information.
The usual duration of treatment for all patients is seven to ten days. In difficult and complicated infections, a longer course of therapy may be necessary. In such cases monitoring of renal, auditory, and vestibular functions is recommended, since toxicity is more apt to occur with treatment extended for more than ten days. Dosage should be reduced if clinically indicated.
The intravenous administration of Gentamicin may be particularly useful for treating patients with bacterial septicemia or those in shock. It may also be the preferred route of administration for some patients with congestive heart failure, hematologic disorders, severe burns, or those with reduced muscle mass. For intermittent intravenous administration in adults, a single-dose of Gentamicin may be diluted in 50 to 200 mL of sterile isotonic saline solution or in a sterile solution of 5% dextrose in water, in infants and children, the volume of diluent should be less. The solution may be infused over a period of one-half to two hours.
The recommended dosage for intravenous and intramuscular administration is identical.
Gentamicin should not be physically premixed with other drugs, but should be administered separately in accordance with the recommended route of administration and dosage schedule.
Patients with Impaired Renal Function
Dosage must be adjusted in patients with impaired renal function to assure therapeutically adequate, but not excessive blood levels. Whenever possible, serum concentrations of Gentamicin should be monitored. One method of dosage adjustment is to increase the interval between administration of the usual doses. Since the serum creatinine concentration has a high correlation with the serum half-life of Gentamicin, this laboratory test may provide guidance for adjustment of the interval between doses. The interval between doses (in hours) may be approximated by multiplying the serum creatinine level (mg/100 mL) by 8. For example, a patient weighing 60 kg with a serum creatinine level of 2.0 mg/100 mL could be given 60 mg (1 mg/kg) every 16 hours (2 x 8).
In patients with serious systemic infections and renal impairment, it may be desirable to administer the antibiotic more frequently but in reduced dosage. In such patients, serum concentrations of Gentamicin should be measured so that adequate, but not excessive levels result. A peak and trough concentration measured intermittently during therapy will provide optimal guidance for adjusting dosage. After the usual initial dose, a rough guide for determining reduced dosage at eight-hour intervals is to divide the normally recommended dose by the serum creatinine level (). For example, after an initial dose of 60 mg (1 mg/kg), a patient weighing 60 kg with a serum creatinine level of 2.0 mg/100 mL could be given 30 mg every eight hours (60 ÷ 2). It should be noted that the status of renal function may be changing over the course of the infectious process.
It is important to recognize that deteriorating renal function may require a greater reduction in dosage than that specified in the above guidelines for patients with stable renal impairment.
In adults with renal failure undergoing hemodialysis, the amount of Gentamicin removed from the blood may vary depending upon several factors including the dialysis method used. An eight-hour hemodialysis may reduce serum concentrations of Gentamicin by approximately 50%. The recommended dose at the end of each dialysis period is 1 to 1.7 mg/kg depending upon the severity of infection. In children, a dose of 2 mg/kg may be administered.
The above dosage schedules are not intended as rigid recommendations but are provided as guides to dosage when the measurement of Gentamicin serum levels is not feasible.
A variety of methods are available to measure Gentamicin concentrations in body fluids; these include microbiologic, enzymatic and radioimmunoassay techniques.
Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution and container permit.
Other drugs, especially those that affect the kidneys, can interact with Gentamicin resulting in dangerous side effects and/or decreased effectiveness. Do not take any other prescription or over-the-counter medicines, including vitamins, minerals, and herbal products, without first talking to your doctor during treatment with Gentamicin.
Penicillins: Gentamicin is inactivated by solutions containing β-lactam antibiotics (penicillins and cephalosporins) so the 2 drugs should not be administered simultaneously nor should they be combined in the IV fluid. The inactivation of Gentamicin by penicillins may occur in vivo, especially in patients with renal failure who maintain a higher level of the penicillin for a longer period of time. Therefore, when Gentamicin and penicillins are used together in patients with renal failure, the time of administration of each drug should be staggered so that several hours separate each infusion.
Diuretics: Potent diuretics eg, ethacrynic acid or frusemide may potentiate the ototoxic effects of Gentamicin.
Other Neurotoxic and/or Nephrotoxic Agents: Since the ototoxic or nephrotoxic effects of Gentamicin may be additive, avoid concurrent or sequential use of other neurotoxic and/or nephrotoxic antibiotics, including other aminoglycosides, polymyxin B, colistin, cisplatin, vancomycin, amphotericin, clindamycin and cephalosporins.
Neuromuscular-Blocking Agents: Respiratory paralysis and prolongation of neuromuscular blockade may occur if a neuromuscular-blocking agent eg, suxamethonium (succinylcholine), tubocurarine, decamethonium, halogenated hydrocarbon inhalation anaesthetics, opioid analgesics or massive transfusions with citrated anticoagulated blood are administered to a patient receiving Gentamicin.
Vitamin K: Gentamicin may inhibit the action of IV vitamin K upon the synthesis of clotting factors.
Potential Interactions: In vitro synergism and antagonism have been found between various antineoplastic agents and aminoglycosides.
Incompatibilities. When Gentamicin is used in combination with any other drug, mixing the drugs before administration should be avoided at all costs.
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Information checked by Dr. Sachin Kumar, MD Pharmacology