Ampicillin

Overview

Ampicillin is a bactericidal aminopenicillin antibiotic used in dogs and cats primarily in parenteral formulations due to its poor oral absorption. Although chemically similar to amoxicillin and sharing the same antimicrobial spectrum, ampicillin is generally not recommended for oral administration in small animals, as amoxicillin provides superior and more reliable oral bioavailability.

In clinical practice, ampicillin is most commonly administered by injection when an aminopenicillin is indicated, particularly for hospitalized patients or when oral therapy is not feasible. Its antimicrobial activity includes many penicillin-susceptible gram-positive organisms, such as streptococci and enterococci, as well as selected gram-negative bacteria. However, its effectiveness is limited against beta-lactamase–producing strains, which restricts its empirical use as a sole agent in many infections.

Mechanism of Action (MOA): Ampicillin acts by inhibiting bacterial cell wall synthesis through binding to penicillin-binding proteins, resulting in cell lysis and death. It is a time-dependent antimicrobial, meaning efficacy depends on maintaining drug concentrations above the minimum inhibitory concentration (MIC) for an adequate duration. Like other aminopenicillins, ampicillin is susceptible to inactivation by beta-lactamase enzymes produced by certain bacteria.

Indications

In dogs and cats, ampicillin is primarily used as a parenteral aminopenicillin when injectable therapy is required or when oral administration is not appropriate. Because oral absorption is poor in small animals, its use by mouth has largely been replaced by amoxicillin. Injectable ampicillin remains clinically valuable for susceptible infections, particularly in hospitalized or systemically ill patients.

• Systemic bacterial infections: Used for infections caused by penicillin-susceptible organisms, including streptococci, enterococci, and selected staphylococci, provided beta-lactamase production is not present.
• Serious enterococcal infections: Effective at higher doses against penicillin-susceptible Enterococcus spp; often combined with an aminoglycoside to achieve synergistic bactericidal activity in severe infections.
• Empirical therapy for sepsis: Commonly used as part of combination therapy with agents that provide enhanced gram-negative coverage (eg, aminoglycosides or fluoroquinolones) when broad-spectrum parenteral coverage is needed.
• Leptospirosis (dogs): Used as an alternative parenteral treatment when doxycycline cannot be administered due to vomiting or intolerance.
• Perioperative prophylaxis: Occasionally used for surgical prophylaxis when an aminopenicillin is indicated, although efficacy is limited by the prevalence of beta-lactamase–producing bacteria.

Dosage (Reference)

Dog

In dogs, ampicillin is most commonly administered by injectable routes because oral absorption is poor. Dosing depends on the severity of infection and whether the drug is used as definitive or empirical therapy. Injectable sodium formulations are required for IV use.

Cat

In cats, ampicillin is used primarily as an injectable antibiotic due to unreliable oral absorption. Dosing principles are similar to dogs, with careful attention to renal function and severity of infection.

Warnings & Precautions

Ampicillin is generally well tolerated in dogs and cats, but appropriate patient selection, formulation choice, and dose adjustment are essential to minimize adverse effects and treatment failure. Most precautions are related to hypersensitivity reactions, renal function, and improper route of administration.

• Hypersensitivity reactions: Contraindicated in dogs and cats with a known history of hypersensitivity to penicillins. Cross-reactivity may occur in patients allergic to other beta-lactam antibiotics, including cephalosporins and carbapenems.
• Formulation-specific precautions: Ampicillin trihydrate (Polyflex®) must NOT be administered intravenously due to a high risk of anaphylaxis and sudden death. Only ampicillin sodium formulations are appropriate for IV administration.
• Renal impairment: Use cautiously in dogs and cats with reduced renal function. Decreased renal clearance may lead to increased plasma concentrations and prolonged elimination; dosage interval adjustments may be required.
• Severe systemic infections: Trihydrate injectable formulations achieve lower serum concentrations than sodium salts and may be inadequate when higher MICs are required; sodium formulations are preferred for serious infections.
• Seizure threshold: High doses of penicillins have been associated with CNS excitation and a lowered seizure threshold in humans; veterinary significance is unclear, but caution is advised when used with other drugs that may lower seizure threshold.
• Route of administration: Injectable products labeled for IM or SC use should not be administered IV unless specifically formulated for IV injection.
• Use during pregnancy and lactation: Ampicillin crosses the placenta and is excreted in low concentrations in milk. Use only when the potential maternal benefit outweighs possible risks to the fetus or nursing offspring.

Drug Interactions

The following drug interactions with ampicillin have been reported or are theoretical in dogs and cats. Most are related to altered antimicrobial activity, changes in drug absorption or elimination, or increased risk of adverse effects. Concurrent use is not always contraindicated, but careful monitoring and dose adjustment may be required.

• Aminoglycosides (eg, gentamicin, amikacin): Beta-lactam antibiotics can inactivate aminoglycosides when mixed in vitro and potentially in vivo in patients with renal impairment. When used together, administer separately and monitor renal function closely.
• Bacteriostatic antimicrobials (eg, tetracyclines, macrolides, chloramphenicol, sulfonamides): In vitro antagonism with beta-lactam antibiotics has been described, which may reduce bactericidal activity; clinical significance remains uncertain.
• Probenecid: Competitively inhibits renal tubular secretion of penicillins, leading to increased serum concentrations and prolonged elimination half-life of ampicillin.
• Methotrexate: Ampicillin may reduce renal excretion of methotrexate, increasing the risk of toxicity; use together with caution.
• Mycophenolate: Concurrent administration may reduce serum concentrations or therapeutic effects of mycophenolate.
• Warfarin: Concurrent use may decrease vitamin K production by intestinal flora and increase the risk of bleeding.
• Proton pump inhibitors (eg, omeprazole, pantoprazole): May reduce oral absorption of ampicillin, further limiting the effectiveness of oral administration.
• Allopurinol: Increased incidence of skin rashes has been reported in humans; veterinary relevance in dogs and cats is unknown.

Side Effects & Overdose

Side Effects

Adverse effects associated with ampicillin in dogs and cats are generally infrequent and usually mild. Most reactions are not dose-dependent and are related to hypersensitivity or gastrointestinal disturbance.

• Gastrointestinal effects: Anorexia, vomiting, and diarrhea may occur, particularly with oral administration. Alteration of the normal gut microbiota can lead to antibiotic-associated diarrhea or superinfection.
• Hypersensitivity reactions: Unpredictable, non–dose-related reactions may include skin rashes, fever, eosinophilia, lymphadenopathy, hematologic abnormalities, or anaphylaxis. These reactions may occur even after previous uneventful exposure.
• Neurologic effects: Ataxia and other signs of neurotoxicity have been reported in dogs receiving very high doses or prolonged therapy.
• Hematologic effects: Rare cases of anemia, leukopenia, thrombocytopenia, or neutropenia have been reported with penicillins.
• Respiratory and cardiovascular signs (dogs): Tachypnea, dyspnea, edema, and tachycardia have been reported infrequently.
• Hepatic effects: Penicillins are not considered hepatotoxic, but transient elevations in liver enzymes have been reported.

Overdose

Ampicillin has a wide margin of safety in dogs and cats. Acute overdoses are most likely to result in gastrointestinal upset, but more serious effects may occur with very high parenteral doses or in patients with impaired renal function.

• Gastrointestinal signs: Vomiting and diarrhea are the most common findings following oral overdose.
• CNS effects: High parenteral doses, particularly in animals with renal disease, may lead to CNS excitation, tremors, ataxia, or seizures.
• Management: Treatment is primarily supportive and symptomatic, including discontinuation of the drug, fluid therapy if indicated, and monitoring of neurologic and renal function.

Key Notes

Practical clinical considerations to optimize the effective use of ampicillin in dogs and cats, focusing on real-world decision making and treatment planning:

• Route selection is critical: Due to unreliable oral absorption, ampicillin is best reserved for injectable use in dogs and cats when an aminopenicillin is specifically indicated.
• Time-dependent activity: Clinical success depends on maintaining drug concentrations above the MIC for an adequate duration; appropriate dosing intervals are more important than peak concentration.
• Culture and susceptibility testing: Whenever possible, therapy should be guided by bacterial culture and susceptibility results to avoid ineffective treatment against resistant organisms.
• Combination therapy rationale: Ampicillin is commonly paired with other antimicrobials to broaden coverage or enhance bactericidal activity in severe infections, rather than being relied upon as monotherapy.
• Hospital-based use: Most clinical applications involve inpatient or closely monitored patients, where parenteral administration and frequent reassessment are feasible.
• Transition to oral therapy: Once clinical improvement is achieved and oral treatment becomes appropriate, switching to amoxicillin is often preferred for continued outpatient management.
• Antimicrobial stewardship: Use should be justified by a clear clinical indication, as inappropriate or prolonged use may contribute to antimicrobial resistance.