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

I. Characteristics of Gentamicin

1. It belongs to the class of bactericidal antibiotics.
2. It is a broad-spectrum aminoglycoside antibiotic.
3. It exhibits a significant post-antibiotic effect (PAE) against Gram-negative bacteria and Gram-positive cocci, meaning it has a sustained inhibitory effect on bacterial growth even at sub-inhibitory concentrations. Bacterial growth remains suppressed for a certain period after the drug is completely eliminated or when the drug concentration falls below the minimum inhibitory concentration (MIC).
4. It is a concentration-dependent antibacterial drug. Recent studies have shown that the antibacterial activity of gentamicin depends on the effective concentration of the antibiotic at 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 MIC, thereby 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 of the total daily dose can reduce the incidence of adaptive resistance and ototoxicity/nephrotoxicity, and can also minimize stress responses caused by restraining pigs for oral administration or injection.
5. It is minimally absorbed orally, resulting in high concentrations within the intestinal tract. Therefore, it can be used as a medication for intestinal infections, treating conditions like neonatal piglet diarrhea (scours) and enteritis.
6. It is rapidly and completely absorbed after parenteral administration (injection). Peak serum concentrations are achieved within 0.5-1 hour after intramuscular injection, with a bioavailability of up to 90%. Increasing the dosage can elevate serum concentrations and appropriately prolong the duration of action.
7. It is an alkaline compound, readily soluble in water, and relatively stable. Its antibacterial activity is strongest in a slightly alkaline environment.
8. Gentamicin can have certain toxic side effects if used in excessively high doses or for prolonged periods.
9. Bacteria can easily develop resistance to gentamicin, but the prevalence of resistant strains is significantly lower than with streptomycin or kanamycin. Adequate dosage or combination with other antibiotics, and avoiding topical application, can reduce or prevent resistance development. Once resistance occurs, immediate discontinuation of the drug allows bacteria to regain sensitivity. Incomplete cross-resistance exists between different aminoglycoside drugs.

II. Drug Interactions

1. The combined use of gentamicin with β-lactam antibiotics (such as penicillins or cephalosporins) usually has a synergistic effect against various Gram-negative bacteria, including Pseudomonas aeruginosa. Synergistic effects are also observed against Gram-positive bacteria. For example, gentamicin combined with penicillin acts synergistically against Streptococcus, 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 cause damage to the bacterial cell wall, which can increase the uptake of gentamicin by the bacterial cell membrane, as cell wall damage makes it easier for the drug to access the cell membrane.
2. Combined use with trimethoprim-sulfonamide also has a synergistic effect against Escherichia coli and Klebsiella pneumoniae.
3. When treating severe systemic infections, gentamicin can be used in combination with dexamethasone, florfenicol, etc., resulting in higher cure rates.
4. The antibacterial activity of gentamicin is enhanced in an alkaline environment. Combining it with alkaline drugs (such as sodium bicarbonate, aminophylline, etc.) can increase its antibacterial potency, but the toxicity may also be correspondingly increased. However, when the pH exceeds 8.4, the antibacterial effect may decrease.
5. It is contraindicated to mix with vitamin C, as it may reduce efficacy.

III. Indications

IV. Dosage and Administration

The potency of this product is calculated by weight, with 1 gram equivalent to 1,000,000 units.

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

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

The following is an empirical formula for treating piglet diarrhea, which the author has developed and refined through ten years of repeated trials. The dosage is slightly higher than the official recommendations but has shown good clinical results. Interested parties may consider trying it:

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

To reduce stress caused by restraining pigs, it can also be administered orally 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 used intramuscularly. The intramuscular single dose is 6-8 mg per kilogram of body weight, which is equivalent to 0.15-0.2 ml/kg body weight of a 4% gentamicin injection, twice daily, for 2-3 days. If administered intramuscularly 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 solution for intraperitoneal injection.

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

V. Precautions

1. Although gentamicin can be used in combination with penicillins or cephalosporins, it should not be diluted with gentamicin injection solution or mixed in the same syringe or infusion bottle for injection. Doing so may reduce the efficacy of gentamicin. Penicillin and gentamicin should be administered separately by intramuscular injection or intravenous infusion.
2. It is best not to administer gentamicin orally to newborn piglets that have not yet consumed colostrum to prevent neonatal piglet diarrhea (scours). The author once trusted articles claiming that administering 60,000 units of gentamicin orally before colostrum intake could prevent scours. However, upon trying this, 6 out of 10 piglets successively died, with severe ulcers at the bottom of the stomach being the cause of death. Learning from this experience, the author suggests that it is best not to administer gentamicin orally to prevent scours in piglets that have not yet consumed colostrum and have empty stomachs, to avoid gastric ulcers and unnecessary losses.
3. This product should not be used in combination with streptomycin, kanamycin, amikacin, neomycin, or other aminoglycoside drugs to prevent enhanced toxicity.
4. This product should not be administered by rapid intravenous injection as it can cause respiratory depression.