Amoxicillin

Antibiotic Aminopenicillin

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 10-20 mg/kg q8-12h Duration: 5-7 days or as clinically indicated
Notes: Administer with food to reduce GI upset. For urinary tract infections, lower doses may be effective.
Cat PO 10-20 mg/kg q12h Duration: 5-7 days or as clinically indicated
Notes: Use with caution in cats with renal impairment.
Horse PO 10-20 mg/kg q8-12h Duration: 5-7 days or as clinically indicated
Notes: Oral absorption is variable; consider parenteral therapy for severe infections.
Horse IM/IV 10-20 mg/kg q12h Duration: 5-7 days or as clinically indicated
Notes: IV administration should be slow. IM injections may cause pain.
Cattle IM/SC 10-20 mg/kg q12-24h Duration: 3-5 days or as clinically indicated
Notes: Observe withdrawal times for milk and meat.
Sheep/Goat PO/IM/SC 10-20 mg/kg q12-24h Duration: 3-5 days or as clinically indicated
Notes: Use extra-label with caution; observe withdrawal times.
Rabbit PO 10-20 mg/kg q12h Duration: 5-7 days or as clinically indicated
Notes: May cause GI dysbiosis; monitor for diarrhea.
Poultry PO (drinking water) 10-20 mg/kg q24h Duration: 3-5 days
Notes: Ensure adequate water intake. Withdrawal times apply.

Clinical Indications & Species Uses

General Indications
  • Treatment of bacterial infections caused by susceptible gram-positive and gram-negative organisms.
Dog (Canine)
  • Treatment of bacterial infections caused by susceptible organisms: skin and soft tissue infections, urinary tract infections, respiratory tract infections, gastrointestinal infections, periodontal infections, and prophylaxis for dental procedures.
Cat (Feline)
  • Treatment of bacterial infections: urinary tract infections, respiratory tract infections, skin and soft tissue infections, and abscesses.
Horse (Equine)
  • Treatment of respiratory tract infections (e.g., Streptococcus equi), skin and soft tissue infections, urinary tract infections, and septicemia (when susceptible).
Cattle (Bovine)
  • Treatment of respiratory tract infections (e.g., pasteurellosis), enteric infections, metritis, and foot rot caused by susceptible organisms.
Small Ruminants (Sheep / Goat)
  • Treatment of respiratory infections, enteritis, mastitis, and skin infections in sheep and goats.
Rabbit & Small Mammals
  • Treatment of respiratory infections (e.g., Pasteurella multocida), urinary tract infections, and skin infections.
Avian & Poultry
  • Treatment of colibacillosis, salmonellosis, and respiratory infections in chickens, turkeys, and other poultry.
Exotic & Other Species
  • Treatment of bacterial infections in reptiles (e.g., abscesses, respiratory infections), small mammals (e.g., guinea pigs, hamsters), and exotic pets.

Pharmacology & Mechanism of Action

Drug Class: Antibiotic | Pharmacological Group: Aminopenicillin

Mechanism of Action: Amoxicillin is a beta-lactam antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs) and interfering with the transpeptidation reaction. This leads to weakening of the peptidoglycan layer, causing cell lysis and death, particularly in actively dividing bacteria. It is bactericidal and time-dependent.

Pharmacodynamics: Amoxicillin has a broad spectrum of activity against many gram-positive and some gram-negative bacteria. It is effective against Streptococcus spp., Staphylococcus spp. (non-beta-lactamase producing), Enterococcus spp., Listeria monocytogenes, and some gram-negative organisms such as Escherichia coli, Salmonella spp., and Pasteurella spp. It is not effective against beta-lactamase-producing organisms (e.g., Staphylococcus aureus resistant strains, many Enterobacteriaceae) and is generally inactive against Pseudomonas aeruginosa and anaerobes like Bacteroides fragilis.

⚡ Pharmacokinetics Summary

Absorption: Amoxicillin is well absorbed after oral administration in most species, with bioavailability ranging from 60-80% in dogs and cats. In horses, oral absorption is variable and less reliable. It is acid-stable and can be given with or without food, though food may delay absorption.
Distribution: Amoxicillin is widely distributed in body tissues and fluids, including lung, liver, kidney, muscle, and pleural, peritoneal, and synovial fluids. It crosses the blood-brain barrier only when meninges are inflamed. It crosses the placenta and is excreted in milk.
Metabolism: Amoxicillin undergoes minimal hepatic metabolism; the majority is excreted unchanged in the urine via tubular secretion and glomerular filtration. A small portion is metabolized to penicilloic acid.
Excretion: Amoxicillin is primarily excreted renally, with high concentrations in urine. In ruminants, it is also excreted in bile and undergoes enterohepatic recycling. In lactating animals, it is excreted in milk.
Half-Life: Dog: 0.5-1 hour; Cat: 0.5-1 hour; Horse: 0.5-1.5 hours; Cattle: 1-2 hours; Sheep/Goat: 1-2 hours; Rabbit: 1-2 hours; Poultry: 1-2 hours.
Bioavailability: Oral: 60-80% in dogs and cats; 40-60% in horses (unreliable); 50-70% in cattle; 60-80% in small ruminants; 50-70% in rabbits; 60-80% in poultry. Parenteral (IM/SC) is 100% bioavailable.
Protein Binding: Amoxicillin is approximately 17-20% protein-bound in dogs and cats, and about 20% in horses and ruminants.

Available Formulations & Strengths

Oral Tablet 50 mg, 100 mg, 200 mg, 400 mg (PO)
Oral Capsule 250 mg, 500 mg (PO)
Oral Suspension 50 mg/mL, 100 mg/mL (PO)
Injectable Solution 100 mg/mL, 200 mg/mL (IM/SC/IV)
Powder for Oral Solution Various (e.g., 100 g per packet) (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to penicillins or cephalosporins
  • Use in rabbits, guinea pigs, hamsters, and other small herbivores (oral administration may cause fatal enteritis due to disruption of normal gut flora)
  • Use in animals with severe renal impairment (dose adjustment may be necessary)
  • Concurrent use with bacteriostatic antibiotics (e.g., tetracyclines, chloramphenicol) may antagonize bactericidal effect
Warnings & Clinical Precautions:
  • Use with caution in animals with a history of allergic reactions to beta-lactam antibiotics
  • In horses, oral administration may be associated with diarrhea and colitis; monitor closely
  • In ruminants, oral administration may disrupt rumen flora; use parenteral routes when possible
  • Prolonged use may result in overgrowth of non-susceptible organisms (e.g., Candida, Clostridium difficile)
  • Use in food animals must comply with withdrawal times and extra-label drug use regulations
  • In cats, high doses may cause neurologic signs (excitement, ataxia) – use with caution
  • In reptiles, amoxicillin may be less effective; consider alternative antibiotics

Adverse Effects & Reactions

Common:

  • Gastrointestinal upset (vomiting, diarrhea, anorexia)
  • Allergic reactions (skin rash, urticaria)
  • Injection site pain or swelling

Serious / Severe:

  • Anaphylaxis (rare but potentially fatal)
  • Clostridium difficile-associated diarrhea (especially in horses)
  • Acute interstitial nephritis (rare)
  • Hepatotoxicity (rare)
  • Neurologic signs (seizures, ataxia) with high doses or renal impairment

Rare:

  • Blood dyscrasias (neutropenia, thrombocytopenia)
  • Stevens-Johnson syndrome
  • Superinfection with resistant organisms

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Probenecid Decreases renal tubular secretion of amoxicillin, increasing its plasma concentration and duration of action. Moderate
Bacteriostatic antibiotics (e.g., tetracyclines, chloramphenicol, macrolides) May antagonize the bactericidal effect of amoxicillin; avoid concurrent use. High
Aminoglycosides (e.g., gentamicin) Synergistic effect against some bacteria; however, both are nephrotoxic and ototoxic, so monitor renal function. Moderate
Methotrexate Amoxicillin may reduce renal clearance of methotrexate, increasing its toxicity. Moderate
Oral contraceptives (in humans, not typically used in animals) May reduce efficacy of oral contraceptives; not relevant in veterinary medicine. Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Gastrointestinal signs (vomiting, diarrhea)
  • Neurologic signs (tremors, seizures, ataxia) especially with high doses or renal impairment
  • Allergic reactions (urticaria, angioedema, anaphylaxis)
  • Electrolyte imbalances (with sodium-containing formulations)

Emergency Treatment Protocol: Treatment is primarily symptomatic and supportive. Induce vomiting if recent ingestion and animal is stable. Administer activated charcoal to reduce absorption. Provide IV fluids to maintain hydration and promote renal excretion. Control seizures with diazepam or barbiturates. For anaphylaxis, administer epinephrine and corticosteroids. Monitor renal function and electrolyte status.

Food Animal Withdrawal Times

🥩 Meat: 7 days🥛 Milk: 3 days🥚 Eggs: 3 days

Withdrawal times vary by country and formulation. For cattle, sheep, and goats, meat withdrawal is typically 7 days and milk 3 days. For poultry, meat withdrawal is 7 days and eggs 3 days. Always follow label or veterinary guidance.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F); excursions permitted between 15-30°C (59-86°F).

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from moisture. Oral suspensions should be refrigerated after reconstitution and discarded after 14 days. Injectable solutions should be stored away from light and not frozen.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: FDA-approved for use in dogs, cats, and certain food animals (e.g., cattle, swine, poultry) in various formulations. Some formulations are approved for horses.

Extra-Label (Off-Label) Use: In the US, extra-label use of amoxicillin in food animals is permitted under AMDUCA (Animal Medicinal Drug Use Clarification Act) only with a valid veterinary-client-patient relationship and a written prescription. It is prohibited to use amoxicillin in feed for food animals (except under specific conditions).

Clinical Pearls & Practice Notes

💡 Clinical Insights: Amoxicillin is a broad-spectrum penicillin commonly used in veterinary practice for a variety of infections. It is often combined with clavulanic acid (amoxicillin/clavulanate) to overcome beta-lactamase resistance. In dogs and cats, it is a first-line antibiotic for urinary tract infections, skin infections, and respiratory infections. In horses, oral amoxicillin is poorly absorbed and may cause diarrhea; parenteral administration is preferred. In ruminants, oral administration is not recommended due to disruption of rumen flora; parenteral routes are preferred. In rabbits and other small herbivores, oral amoxicillin is contraindicated due to risk of fatal enteritis. Always culture and sensitivity testing when possible to ensure efficacy. Monitor for adverse effects, especially gastrointestinal upset and allergic reactions. Adjust dose in renal impairment. For food animals, adhere to withdrawal times.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)