Amoxicillin Binds to penicillin-binding proteins involved in bacterial cell wall synthesis, thereby decreasing cell wall strength and rigidity, affecting cell division, growth and septum formation. Acts in a time-dependent fashion.

Effective against:

Gram-positive organisms: Streptococcus spp., non-beta-lactamase producing Staphylococcus spp.
Gram-negative organisms: Escherichia coli, Haemophilus spp., and Proteus mirabilis.
Anaerobes: Clostridium spp., Bacteroides spp. (except beta-lactamase-producing strains).

Uses of Amoxicillin

Dogs and Cats: Bacterial cystitis, respiratory infections, soft tissue infections, and gastrointestinal infections.
Birds: Treatment of susceptible bacterial infections.
Rabbits and Rodents: Avoid use due to the risk of severe GI distress and enterotoxemia.
Reptiles: Broad-spectrum coverage against susceptible infections (requires combination therapy for Gram-negative coverage).

Dose of Amoxicillin

Dose of Amoxicillin in Dogs and Cats:

Parenteral:
- 7 mg/kg i.m. q24h.
- 15 mg/kg i.m. q48h (for depot preparations).
Oral:
- 10 mg/kg p.o. q8–12h.
- 11–15 mg/kg p.o. q8h for bacterial cystitis. Studies suggest clavulanic acid may not always be necessary, even for beta-lactamase-producing bacteria.

Dose of Amoxicillin in Birds:

100–125 mg/kg p.o. q12h. Adjust dose frequency for species-specific metabolism.

Dose of Amoxicillin in Reptiles:

22 mg/kg p.o. q12–24h. Combine with aminoglycosides for synergistic action against resistant Gram-negative organisms.

Dose of Amoxicillin in Cattle:

7–10 mg/kg i.m. q24h for systemic infections (extra-label).
62.5 mg per affected teat for mastitis (intramammary infusion).

Dose of Amoxicillin in Ferrets:

10–20 mg/kg p.o. q12h. Often used in combination with metronidazole and bismuth subsalicylate for Helicobacter spp. infections.

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Side Effects of Amoxicillin

Common side effects:
- Nausea and diarrhea.
- Skin rashes (type I IgE-mediated hypersensitivity reactions).
Less common side effects:
- Elevated liver enzymes.
- Neurotoxicity (e.g., ataxia) at high doses or with prolonged use.
- Antibiotic-associated colitis (increased risk in animals with altered gut flora).

Contraindications of Amoxicillin

Avoid oral administration in critically ill patients due to unreliable GI absorption.
Do not use in animals hypersensitive to beta-lactams (e.g., penicillins or cephalosporins).
Avoid in species prone to severe GI reactions, such as rabbits, guinea pigs, hamsters, and chinchillas.

Some Notes:

Resistance:
- Resistance is common among beta-lactamase-producing bacteria such as certain strains of E. coli and Staphylococcus aureus.
- Gram-negative organisms like Pseudomonas spp., Proteus spp., and Klebsiella spp. are typically resistant.
Excretion:
- Amoxicillin is excreted via bile and urine, achieving high urinary concentrations, making it effective for urinary tract infections.
Administration:
- Oral amoxicillin can be given with or without food.
- Refrigerate oral suspensions after reconstitution and discard after 7 days or if the solution darkens.
Interactions:
- Avoid concurrent use with bacteriostatic antibiotics (e.g., tetracycline, erythromycin, chloramphenicol) as antagonistic effects may occur.
- Do not mix in the same syringe as aminoglycosides, although synergistic effects may occur when used concurrently via separate routes.
Efficacy:
- Maintaining plasma concentrations above the minimum inhibitory concentration (MIC) is crucial for therapeutic success. Adhere to regular dosing schedules to avoid resistance and ensure efficacy.