Gentamicin Characteristics, Interactions, Indications, Usage, Dosage, and Precautions

I. Characteristics of Gentamicin

1. Belongs to bactericidal antibiotics.
2. Belongs to the broad-spectrum aminoglycoside class.
3. Exhibits a significant post-antibiotic effect (PAE) against Gram-negative bacteria and Gram-positive cocci, meaning it has a sustained inhibitory effect even at sub-inhibitory concentrations, after the drug is completely eliminated, or when drug concentrations fall below the minimum inhibitory concentration (MIC). Bacterial growth remains suppressed for a certain period.
4. Belongs to concentration-dependent antibacterial drugs. Recent studies indicate that the antibacterial activity of gentamicin depends on the effective concentration of the antibiotic on the cell membrane. With the same daily dose, single daily administration can achieve a higher peak serum concentration than multiple daily administrations, increasing the ratio of peak concentration to minimum inhibitory concentration and thus significantly improving antibacterial activity and clinical efficacy.
   Furthermore, the post-antibiotic effect (PAE) of gentamicin against pathogenic bacteria is also concentration-dependent. Single daily administration can reduce the incidence of adaptive resistance and ototoxicity and nephrotoxicity, as well as decrease stress responses caused by capturing pigs for oral administration or injection.
5. Oral administration of this product is poorly absorbed, resulting in high concentrations in the intestinal tract, making it suitable for treating intestinal infections such as yellow and white scours and enteritis in piglets.
6. Absorption of this product is rapid and complete after injection. Peak serum concentrations are reached within 0.5-1 hour after intramuscular injection, and bioavailability can reach 90%. Increasing the dosage can raise serum concentrations and appropriately prolong the duration of action.
7. This product is a basic compound, readily soluble in water, and relatively stable. Its antibacterial activity is strongest in a slightly alkaline environment.
8. Excessive dosage or prolonged use of gentamicin can lead to certain toxic side effects.
9. Bacteria can easily develop resistance to this product, although resistant strains are less common than with streptomycin or kanamycin. Sufficient dosage or combination with other antibiotics, and avoiding local application, can reduce or prevent the development of resistance. Once resistance occurs, immediate discontinuation of the drug allows bacteria to regain sensitivity. Incomplete cross-resistance exists among different aminoglycoside drugs.

II. Drug Interactions

1. Gentamicin combined with β-lactam antibiotics (such as penicillins or cephalosporins) usually has a synergistic effect against various Gram-negative bacteria, including Pseudomonas aeruginosa, and also against Gram-positive bacteria. For example, gentamicin in combination with penicillin acts synergistically against streptococci, with enzyme-resistant semi-synthetic penicillins against Staphylococcus aureus, with cephalosporins against Klebsiella pneumoniae, with penicillin G or ampicillin against Listeria monocytogenes, and with carbenicillin against Pseudomonas aeruginosa. This is because β-lactam drugs damage bacterial cell walls, which can increase the uptake of gentamicin by the bacterial cell membrane, as cell wall damage makes it easier for the drug to reach the cell membrane.
2. Concurrent use with trimethoprim-sulfonamide also has a synergistic effect against Escherichia coli and Klebsiella pneumoniae.
3. When treating severe systemic infections, this product can be used in combination with dexamethasone, florfenicol, etc., resulting in a higher cure rate.
4. The antibacterial activity of this product is enhanced in an alkaline environment. Combined use with alkaline drugs (such as sodium bicarbonate, aminophylline, etc.) can increase antibacterial activity, but toxicity may also be correspondingly increased. When the pH value exceeds 8.4, the antibacterial effect may decrease.
5. It is forbidden to mix with vitamin C, as it may reduce efficacy.

III. Indications

Gentamicin has good efficacy in treating severe infections caused by susceptible bacteria. It is mainly used clinically to control and treat local and systemic infections caused by drug-resistant Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Proteus spp., Pasteurella spp., Salmonella spp., and other aerobic Gram-negative bacteria, such as sepsis, urogenital tract infections, respiratory tract infections (pneumonia, bronchopneumonia), gastrointestinal infections (including peritonitis), mastitis, osteoarticular infections, and skin and soft tissue infections.

IV. Usage and Dosage

The potency of this product is calculated by weight, with 1 gram equal to 1 million units.

1. Oral administration: Daily dose of 10-15 mg per kilogram of body weight, divided into 2-3 administrations per day, for 2-3 consecutive days.
2. Intramuscular injection: Single dose of 2-4 mg per kilogram of body weight, twice a day, for 2-3 consecutive days.

The above are the dosages recommended by the official “Veterinary Drug Use Guidelines.”

The following is an empirical formula for treating piglet diarrhea, summarized by the author after ten years of repeated trials. The dosage is slightly higher than the official recommended dosage, and the clinical effect is good. Interested parties may wish to try it:

Gentamicin powder 5.0 grams, trimethoprim 1.0 gram, scopolamine butylbromide tablets 20 tablets (5 mg/tablet), oral rehydration salt 13.75 grams, cooled boiled water 500 ml. For oral administration, a single dose is 1 ml per kilogram of body weight, twice a day, for 2-3 consecutive days.

To reduce stress caused by capturing pigs, oral administration can also be given once daily at a dose of 2.0 ml per kilogram of body weight.

If piglets weigh more than 10 kg and oral administration is inconvenient, gentamicin injection can be given intramuscularly. Single intramuscular injection dose is 6-8 mg per kilogram of body weight, which is equivalent to 0.15-0.2 ml/kg body weight of 4% gentamicin injection, twice a day, for 2-3 consecutive days. If injected once daily, the dose can be doubled.

For severely dehydrated cases, gentamicin injection can also be added to 20-40 ml of 5% glucose saline for intraperitoneal injection.

For severe cases, regardless of oral administration, intramuscular injection, or intraperitoneal injection, the dosage can be appropriately increased at the discretion of the veterinarian.

V. Precautions for Use

1. Although gentamicin can be used in combination with penicillin or cephalosporins, it should not be diluted with gentamicin injection and should not be mixed in the same syringe or infusion bottle for injection, otherwise the efficacy of gentamicin will be reduced. Penicillin and gentamicin can be injected intramuscularly or administered by intravenous drip separately.
2. It is best not to administer gentamicin orally to newborn piglets that have not consumed colostrum to prevent piglet yellow scours. The author once believed articles published in certain newspapers stating that oral administration of 60,000 units of gentamicin to piglets before consuming colostrum could prevent piglet yellow scours. The author tried this, and as a result, 6 out of 10 piglets died successively. The cause of death was severe ulcers at the bottom of the stomach. Once bitten, twice shy. The author suggests that it is best not to administer gentamicin orally to prevent yellow scours in piglets that have not consumed colostrum and whose stomachs are empty, to avoid gastric ulcers and unnecessary losses.
3. This product should not be used in combination with other aminoglycoside drugs such as streptomycin, kanamycin, amikacin, neomycin, etc., to prevent increased toxicity.
4. This product should not be administered by intravenous bolus injection as it can cause respiratory depression.