Amoxicillin
Amoxicillin is a widely prescribed, **broad-spectrum aminopenicillin** antibiotic used in veterinary medicine to treat a variety of bacterial infections. Key pharmacological features include: * **Spectrum of Activity**: Effective against many Gram-positive aerobes, some Gram-negative aerobes (e.g., *E. coli*, *Klebsiella*, *Haemophilus*), and many obligate anaerobes (including *Clostridium* spp.). * **Beta-Lactamase Susceptibility**: Like ampicillin, it is inactivated by beta-lactamase-producing bacteria (e.g., *Staphylococcus aureus*). It is often combined with clavulanate to overcome this resistance. * **Clinical Utility**: It is considered a first-line empiric choice for feline abscesses, susceptible urinary tract infections (UTIs), and soft tissue infections. * **Pharmacokinetic Advantage**: Compared to ampicillin, amoxicillin has significantly better oral bioavailability in monogastric animals, achieving serum levels 1.5 to 3 times higher at equivalent doses.
Mechanism: Amoxicillin is a **time-dependent, bactericidal** antibiotic that disrupts bacterial cell wall synthesis. * **Mechanism**: The beta-lactam ring mimics the D-alanyl-D-alanine terminal of the peptidoglycan precursor. * **Pathway**: Amoxicillin binds to specific **penicillin-binding proteins (PBPs)** (such as transpeptidases, carboxypeptidases, and endopeptidases) located on the inner surface of the bacterial cell membrane โ inhibits the cross-linking of mucopeptide (peptidoglycan) chains โ compromises the structural integrity of the cell wall โ creates an osmotically unstable spheroplast โ results in **cell lysis and death**. * **Efficacy**: It is most effective against actively dividing and growing bacteria.
Dosing by species
- Gram-positive infections ยท 10 mg/kg PO, IM, SC twice daily for at least 2 days after symptoms subside. ยท PO, IM, SC ยท q12h ยท At least 2 days after symptoms subside
- Gram-negative infections ยท 20 mg/kg PO three times daily or IM, SC twice daily for at least 2 days after symptoms subside ยท PO, IM, SC ยท q8h (PO) or q12h (IM, SC) ยท At least 2 days after symptoms subside
- Susceptible UTI's ยท 10-20 mg/kg PO q12h for 5-7 days. ยท PO ยท q12h ยท 5-7 days
- Susceptible systemic infections (bacteremia/sepsis) ยท 22-30 mg/kg IV , IM, SC q8h for 7 days. ยท IV, IM, SC ยท q8h ยท 7 days
- Susceptible orthopedic infections ยท 22-30 mg/kg IV , IM, SC, or PO q6-8h for 7-10 days. ยท IV, IM, SC, PO ยท q6-8h ยท 7-10 days
- Lyme disease ยท 22 mg/kg PO q12h for 21-28 days ยท PO ยท q12h ยท 21-28 days
- Susceptible urinary tract infections ยท 11 mg/kg PO q8h. ยท PO ยท q8h
- Preventative therapy for repeated (>2 per 6 months) urinary tract Gram-positive bacterial infections ยท 20 mg/kg PO once daily before bedtime after the dog has urinated. ยท PO ยท q24h ยท Use only after effective treatment completed using full therapeutic doses.
- Susceptible bacterial infections ยท 7 mg/kg ยท IM ยท q24h
- Susceptible bacterial infections (depot preparations) ยท 15 mg/kg ยท IM ยท q48h ยท For depot preparations only
Routes of administration
Contraindications
- Patients with a history of hypersensitivity to penicillins
- Oral administration in hindgut fermenters (rabbits, guinea pigs, chinchillas, hamsters) due to risk of fatal clostridial enterotoxemia
- Oral administration in patients with septicemia, shock, or grave illness (due to delayed/diminished GI absorption)
Adverse effects
- Gastrointestinal upset (anorexia, vomiting, diarrhea)
- Antibiotic-associated diarrhea and gut flora alteration
- Hypersensitivity reactions (rashes, fever, anaphylaxis)
- Eosinophilia, neutropenia, agranulocytosis, thrombocytopenia, leukopenia, anemias
- Lymphadenopathy
- Neurotoxicity (e.g., ataxia) at very high doses or prolonged use
- Elevated liver enzymes (rare)
- Tachypnea, dyspnea, edema, and tachycardia (reported in dogs)
Drug interactions
- Bacteriostatic antimicrobials (e.g., chloramphenicol, macrolides, tetracyclines, sulfonamides) ยท Potential in vitro antagonism; concurrent use has historically been discouraged, though clinical significance is debated.
- Methotrexate ยท Amoxicillin may decrease the renal excretion of methotrexate, leading to increased levels and potential toxicity.
- Probenecid ยท Competitively blocks the tubular secretion of penicillins, increasing serum levels and prolonging serum half-life.
- Aminoglycosides ยท In vitro inactivation of aminoglycosides by beta-lactams; may falsely decrease aminoglycoside serum concentrations if samples are stored prior to analysis.
- Tetracycline ยท Potential antagonism of bactericidal activity (bacteriostatic vs bactericidal) ยท moderate
- Erythromycin ยท Potential antagonism of bactericidal activity ยท moderate
- Chloramphenicol ยท Potential antagonism of bactericidal activity ยท moderate
- Aminoglycosides (in vitro) ยท Inactivation of aminoglycoside if mixed in the same syringe ยท major
- Aminoglycosides (in vivo) ยท Synergistic antimicrobial effect when used concurrently
Monitoring
- Clinical efficacy (resolution of infection signs)
- Adverse effects (GI upset, signs of hypersensitivity)
Overdose
Acute oral penicillin overdoses are unlikely to cause significant problems other than **gastrointestinal distress** (vomiting, diarrhea, anorexia). In humans, very high dosages of parenteral penicillins, especially in patients with compromised renal function, have induced **CNS effects** (e.g., seizures, ataxia). Treatment is generally supportive and symptomatic.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.