Ampicillin — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	10-25 mg/kg	q8h	Duration: 5-7 days or as directed Notes: Give on an empty stomach (1 hour before or 2 hours after feeding) to enhance absorption.
Dog	IV/IM/SC	10-25 mg/kg	q8h	Duration: 5-7 days or as directed Notes: IV administration is preferred for severe infections. SC may cause pain at injection site.
Cat	PO	10-20 mg/kg	q8h	Duration: 5-7 days or as directed Notes: Give on an empty stomach.
Cat	IV/IM/SC	10-20 mg/kg	q8h	Duration: 5-7 days or as directed Notes: IM may cause pain; SC is acceptable.
Horse	IV	10-25 mg/kg	q6-8h	Duration: 5-7 days or as directed Notes: IM is not recommended due to pain and tissue irritation. Use IV only.
Cattle	IM/SC	10-20 mg/kg	q12-24h	Duration: 3-5 days or as directed Notes: Long-acting formulations may be given q24h. Observe withdrawal times.
Small Ruminants (Sheep/Goats)	IM/SC	10-20 mg/kg	q12-24h	Duration: 3-5 days or as directed Notes: Use with caution; extralabel use may require veterinary oversight.
Rabbit	PO	15-25 mg/kg	q8-12h	Duration: 5-7 days or as directed Notes: Oral administration may be difficult; consider parenteral if needed.
Rabbit	SC/IM	15-25 mg/kg	q8-12h	Duration: 5-7 days or as directed Notes: SC is preferred to IM to avoid muscle damage.
Birds/Poultry	PO (in water)	10-20 mg/kg	q8-12h	Duration: 3-5 days or as directed Notes: Ensure adequate water intake; medicated water should be freshly prepared.
Exotic/Other	Varies	10-25 mg/kg	q8-12h	Duration: 5-7 days or as directed Notes: Dose based on species-specific pharmacokinetics; consult a specialist.

Clinical Indications & Species Uses

General Indications

Treatment of infections caused by ampicillin-susceptible organisms.

Dog (Canine)

Treatment of susceptible bacterial infections: urinary tract infections, respiratory tract infections, skin and soft tissue infections, gastrointestinal infections, septicemia, and meningitis (when susceptible).

Cat (Feline)

Treatment of susceptible bacterial infections: urinary tract infections, respiratory tract infections, skin and soft tissue infections, gastrointestinal infections, and septicemia.

Horse (Equine)

Treatment of susceptible bacterial infections: respiratory tract infections (e.g., strangles), urinary tract infections, wound infections, and septicemia.

Cattle (Bovine)

Treatment of susceptible bacterial infections: respiratory tract infections (e.g., shipping fever), urinary tract infections, metritis, and foot rot.

Small Ruminants (Sheep / Goat)

Treatment of susceptible bacterial infections: respiratory tract infections, urinary tract infections, and skin infections.

Rabbit & Small Mammals

Treatment of susceptible bacterial infections: respiratory tract infections (e.g., Pasteurella multocida), urinary tract infections, and abscesses.

Avian & Poultry

Treatment of susceptible bacterial infections: colibacillosis, salmonellosis, and respiratory infections.

Exotic & Other Species

Treatment of susceptible bacterial infections in reptiles, small mammals, and other exotic species.

Pharmacology & Mechanism of Action

Drug Class: Aminopenicillin | Pharmacological Group: Beta-lactam antibiotic

Mechanism of Action: Ampicillin 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, primarily in actively dividing bacteria. It is bactericidal and time-dependent.

Pharmacodynamics: Ampicillin has a broad spectrum of activity against many gram-positive and some gram-negative bacteria. It is effective against Streptococcus spp., non-beta-lactamase-producing Staphylococcus spp., Enterococcus spp., Listeria monocytogenes, and many strains of Escherichia coli, Salmonella spp., and Pasteurella spp. It is not effective against beta-lactamase-producing organisms (e.g., many Staphylococcus aureus, E. coli, and Haemophilus spp.) or anaerobes like Bacteroides fragilis. Its activity is enhanced by the addition of sulbactam, a beta-lactamase inhibitor.

⚡ Pharmacokinetics Summary

Absorption: Ampicillin is poorly absorbed orally, with bioavailability ranging from 30-50% in dogs and cats. Absorption is decreased when given with food. The oral prodrug, ampicillin trihydrate, is more stable in gastric acid but still has limited absorption. Parenteral administration (IV, IM, SC) provides complete bioavailability.

Distribution: Ampicillin is widely distributed in body tissues and fluids, including pleural, peritoneal, and synovial fluids. It crosses the blood-brain barrier only when the meninges are inflamed. It crosses the placenta and is excreted in milk. Volume of distribution is approximately 0.3-0.5 L/kg in dogs.

Metabolism: Ampicillin undergoes minimal hepatic metabolism. It is primarily eliminated unchanged by the kidneys via glomerular filtration and tubular secretion.

Excretion: Ampicillin is excreted primarily in the urine as unchanged drug. In dogs, approximately 80-90% of an IV dose is recovered in urine within 6 hours. It is also excreted in bile and milk.

Half-Life: Dog: 0.5-1 hour; Cat: 0.5-1 hour; Horse: 0.8-1.5 hours; Cattle: 1-2 hours; Rabbit: 0.5-1 hour.

Bioavailability: Oral: 30-50% in dogs and cats; IM: 80-100% in most species; SC: similar to IM.

Protein Binding: Ampicillin is approximately 20-30% bound to plasma proteins in most species.

Available Formulations & Strengths

Oral Capsule 250 mg, 500 mg (PO)

Oral Tablet 125 mg, 250 mg, 500 mg (PO)

Oral Suspension 125 mg/5 mL, 250 mg/5 mL (PO)

Injectable Solution (as sodium salt) 125 mg/mL, 250 mg/mL (IV/IM/SC)

Injectable Suspension (as trihydrate) 100 mg/mL, 200 mg/mL (IM/SC)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to penicillins or cephalosporins (cross-reactivity).
Use in animals with a history of allergic reactions to beta-lactam antibiotics.
Oral administration in animals with severe gastrointestinal disease (e.g., vomiting, diarrhea) due to poor absorption.
Use in rabbits, guinea pigs, hamsters, and other small herbivores orally due to risk of fatal enterotoxemia (Clostridium difficile overgrowth).

Warnings & Clinical Precautions:

Use with caution in animals with renal impairment; dose adjustment may be necessary.
Prolonged use may result in overgrowth of non-susceptible organisms (e.g., Candida, Pseudomonas).
In horses, IM injection can cause severe tissue irritation and should be avoided.
In ruminants, oral administration may disrupt rumen flora; use parenteral routes when possible.
Extralabel use in food animals must comply with AMDUCA regulations and withdrawal times.
Ampicillin is not effective against beta-lactamase-producing bacteria; consider adding a beta-lactamase inhibitor (e.g., sulbactam) if indicated.

Adverse Effects & Reactions

Common:

Gastrointestinal upset (nausea, vomiting, diarrhea) with oral administration.
Injection site pain or swelling.
Allergic reactions (skin rash, urticaria).

Serious / Severe:

Anaphylaxis (rare but potentially fatal).
Clostridium difficile-associated diarrhea (especially in rabbits and other small herbivores).
Acute interstitial nephritis (rare).
Neurotoxicity (seizures) with high doses in renal failure.

Rare:

Blood dyscrasias (neutropenia, thrombocytopenia).
Hepatotoxicity.
Superinfections (e.g., candidiasis).

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Bacteriostatic antibiotics (e.g., tetracyclines, chloramphenicol, macrolides)	May antagonize the bactericidal effect of ampicillin; avoid concurrent use.	Moderate
Probenecid	Increases ampicillin plasma concentrations by decreasing renal tubular secretion; may be used to prolong therapeutic levels.	Mild
Aminoglycosides (e.g., gentamicin, amikacin)	Synergistic effect against some bacteria; however, do not mix in same syringe or solution as they may inactivate each other.	Moderate
Methotrexate	Ampicillin may reduce renal excretion of methotrexate, increasing toxicity.	Moderate
Oral contraceptives (in humans, but relevant in research)	May reduce efficacy of oral contraceptives; not directly applicable in veterinary medicine.	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Gastrointestinal signs (vomiting, diarrhea).
Neurological signs (tremors, seizures) especially with high IV doses.
Allergic reactions (urticaria, angioedema).

Emergency Treatment Protocol: There is no specific antidote. Treatment is symptomatic and supportive. For recent oral overdose, induce emesis or perform gastric lavage if appropriate. Administer activated charcoal to reduce absorption. For severe neurological signs, use anticonvulsants (e.g., diazepam). Monitor renal function and fluid balance. In cases of anaphylaxis, administer epinephrine and supportive care.

Food Animal Withdrawal Times

🥩 Meat: 6 days🥛 Milk: 48 days

Withdrawal times vary by formulation and country. For cattle, meat withdrawal is typically 6 days for injectable forms; milk withdrawal is 48 hours. For poultry, withdrawal times may be 0-1 days for eggs if approved, but extralabel use may require longer. Always consult local regulations.

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 and light. Oral suspensions should be refrigerated after reconstitution and discarded after 14 days. Injectable solutions should be used within 24 hours after reconstitution if stored at room temperature, or up to 7 days if refrigerated.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Ampicillin is FDA-approved for use in dogs, cats, horses, cattle, and swine in various formulations. However, some formulations may be approved for specific species only.

Extra-Label (Off-Label) Use: In the US, extralabel use of ampicillin in food animals is permitted under AMDUCA only with a valid veterinary-client-patient relationship and must follow withdrawal times. Use in feed is prohibited in some countries.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Ampicillin is a broad-spectrum antibiotic commonly used in veterinary practice. It is particularly useful for infections caused by susceptible gram-positive and gram-negative organisms. Due to its short half-life, frequent dosing is required. Oral absorption is variable and affected by food; therefore, it should be given on an empty stomach. Parenteral administration is preferred for severe infections. In horses, only IV administration is recommended due to the risk of severe muscle irritation. In rabbits and other small herbivores, oral ampicillin is contraindicated due to the risk of fatal enterotoxemia; parenteral administration is safer. Always perform culture and sensitivity testing when possible to ensure susceptibility. Monitor for adverse effects, especially gastrointestinal upset and allergic reactions. In food animals, adhere to withdrawal times to ensure food safety.

References & Literature

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