Ampicillin: veterinary dose and monograph

At a glance

Drug class

Antibiotic

Also known as

Ampi, Polyflex, Roscillin

Species with doses

Dogs, Cats, Cattle, Horses, Pigs, Birds, Ferrets, Reptiles, Rats, Mice, Hedgehogs and Primates

General dose range

10–22 mg/kg IM q8h — check the species tables for the indication

Routes

IV, PO, SC, IM

Last reviewed

1 October 2026

Indications

• Susceptible Gram-positive and Gram-negative infections
• Urinary, soft-tissue and respiratory infections
• Leptospirosis (with supportive care)
• Listeriosis (ruminants, high dose)
• Surgical prophylaxis (sodium salt IV)

Ampicillin dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Dogs

• Sepsis / severe systemic infection (sodium salt): Plumb’s p.96: sepsis 20–40 mg/kg IV q6–8h (Hardie 2000; Greene 2006); meningitis/orthopaedic 22 mg/kg IV, IM or SC q6–8h; leptospiraemic phase 22 mg/kg IV q6–8h (Lunn 2006). Papich 5e: routine infections 10 mg/kg q8h IV, IM or SC.
• Susceptible infection (oral trihydrate): Papich 5e: 20 mg/kg q8h PO. Plumb’s p.96: soft tissue 10–20 mg/kg PO q8h; UTI 25 mg/kg PO q8h (Polzin 2005). Oral absorption is poor and reduced by food — amoxicillin is usually preferred for oral therapy.

Cats

• Susceptible infection: Papich 5e: sodium salt 10 mg/kg q8h IV, IM or SC; trihydrate 10–20 mg/kg q12–24h IM or SC. Plumb’s p.96: systemic infections 7–11 mg/kg IV, IM or SC q8–12h (Greene 2006); sepsis 20–40 mg/kg IV q6–8h.

Cattle

• Susceptible infection (trihydrate, label): Plumb’s p.96 (Beech 1987b): ampicillin trihydrate 11 mg/kg IM q24h. Polyflex label withdrawal at labelled doses, for no more than 7 days: milk 48 h, slaughter 6 days. Papich 5e: trihydrate 11 mg/kg q12h IM — twice-daily dosing is extra-label and needs a longer withdrawal.
• Respiratory infection — high dose (extra-label): Plumb’s p.96 (Hjerpe 1986): ampicillin trihydrate 22 mg/kg SC q12h for respiratory infections, with a suggested 60-day slaughter withdrawal. Milk withdrawal not given — consult FARAD. Papich 5e: sodium salt 22 mg/kg q12h IM or IV (ruminants).

Horses

• Susceptible infection (sodium salt): Papich 5e: ampicillin sodium 22 mg/kg q12h IM or IV. Plumb’s p.96: sodium salt 10–50 mg/kg IV or IM three times daily (Robinson 1987); 11–15 mg/kg IM or IV three to four times daily (Beech 1987a); foals 11 mg/kg q6h (Furr 1999) or 15–30 mg/kg q6–8h (Brumbaugh 1999). Calculator range 10–20 mg/kg IV q6–8h (Papich 22 mg/kg q12h).

Pigs

• Miniature/pet pig — susceptible infection: Carpenter Exotic Animal Formulary 6th ed, Table 12-1 (p.659): miniature pigs 10–20 mg/kg SC/IM/IV q6–8h, or 20 mg/kg SC/IM q8h, or 20–40 mg/kg PO q8h.
• Susceptible infection (trihydrate): Papich 5e: pigs ampicillin trihydrate 15 mg/kg q24h IM. Plumb’s p.96 (Baggot 1983): ampicillin sodium 6–8 mg/kg SC or IM q8h. No withdrawal is given in either reference — follow the product label or FARAD.

Buffalo

No buffalo-specific doses are on file, so buffalo are dosed as cattle.

• Susceptible infection (trihydrate, label): Plumb’s p.96 (Beech 1987b): ampicillin trihydrate 11 mg/kg IM q24h. Polyflex label withdrawal at labelled doses, for no more than 7 days: milk 48 h, slaughter 6 days. Papich 5e: trihydrate 11 mg/kg q12h IM — twice-daily dosing is extra-label and needs a longer withdrawal.
• Respiratory infection — high dose (extra-label): Plumb’s p.96 (Hjerpe 1986): ampicillin trihydrate 22 mg/kg SC q12h for respiratory infections, with a suggested 60-day slaughter withdrawal. Milk withdrawal not given — consult FARAD. Papich 5e: sodium salt 22 mg/kg q12h IM or IV (ruminants).

Birds

• Susceptible infection: Plumb’s p.96: 100 mg/kg IM q4h as the trihydrate in Amazon parrots (Clubb 1986) or q8h (Hoeffer 1995); oral 150–200 mg/kg two to three times daily is poorly absorbed. Ratites 15–20 mg/kg IM twice daily (Jenson 1998).

Rabbits

• Contraindicated: Plumb’s p.96 (Ivey & Morrisey 2000): not recommended in rabbits as it can cause a fatal enteritis (clostridial dysbiosis).

Ferrets

• Susceptible infection: Plumb’s p.96 (Williams 2000): ferrets 5–10 mg/kg IM, SC or IV twice daily.

Guinea pigs

• Contraindicated: Plumb’s p.96 (Adamcak & Otten 2000): guinea pigs, chinchillas and hamsters — do NOT use; may cause enterocolitis.

Reptiles

• Chelonians — susceptible infection: Plumb’s p.96 (Jacobson 2000): chelonians 50 mg/kg IM q12h. For other species Gauvin (1993) lists 3–6 mg/kg PO, SC or IM q12–24h for 2 weeks, of limited value unless combined with an aminoglycoside.

Chinchillas

• Contraindicated: Plumb’s p.96 (Adamcak & Otten 2000): guinea pigs, chinchillas and hamsters — do NOT use; may cause enterocolitis.

Hamsters

• Contraindicated: Plumb’s p.96 (Adamcak & Otten 2000): guinea pigs, chinchillas and hamsters — do NOT use; may cause enterocolitis.

Rats

• Susceptible infection: Plumb’s p.96 (Adamcak & Otten 2000): gerbils, mice and rats 20–100 mg/kg PO, SC or IM q8–12h.

Mice

• Susceptible infection: Plumb’s p.96 (Adamcak & Otten 2000): gerbils, mice and rats 20–100 mg/kg PO, SC or IM q8–12h.

Hedgehogs

• Susceptible infection: Plumb’s p.96 (Smith 2000): hedgehogs 10 mg/kg IM or PO once daily.

Primates

• Susceptible infection (prosimians, lemurs): Carpenter Exotic Animal Formulary 6th ed, Table 13-1 (p.680): prosimians/brown lemurs 10–30 mg/kg SC/IM/IV q6–8h; monkeys 25–50 mg/kg/day IM/IV divided q6–8h; meningitis/septicaemia 150–200 mg/kg/day divided q3–4h.

Contraindications

Penicillin/cephalosporin hypersensitivity. NEVER orally in rabbits, guinea pigs, chinchillas, hamsters — fatal clostridial enterotoxaemia. Caution in horses (colitis risk).

Species-specific warnings

Rabbits: NEVER orally — fatal enterotoxaemia.

Small mammals: Avoid oral penicillins in guinea pigs, chinchillas, hamsters.

Adverse effects

• GI upset — vomiting, diarrhoea
• Hypersensitivity — urticaria, anaphylaxis
• Pain / swelling at IM injection site
• Antibiotic-associated diarrhoea / dysbiosis

Drug interactions

• Bacteriostatic antimicrobials (chloramphenicol, macrolides, tetracyclines, sulfonamides): in vitro antagonism with beta-lactams is documented, so combining has traditionally been discouraged — Plumb's p.95 notes the actual clinical importance is unclear and currently in doubt
• Methotrexate: ampicillin may reduce renal excretion of MTX, raising levels and toxicity risk
• Probenecid: competitively blocks tubular secretion of penicillins, raising serum levels and half-life
• Aminoglycosides: beta-lactams inactivate aminoglycosides in vitro (and in vivo in renal failure) — do not mix in the same syringe or line; aminoglycoside assay results may read falsely low
• Laboratory: may cause FALSE-POSITIVE urine glucose with cupric sulfate methods (Benedict's, Clinitest); glucose-oxidase strips (Tes-Tape, Clinistix) are unaffected

Pregnancy and lactation

Considered safe in pregnancy and lactation (penicillin class). Avoid in lagomorphs/rodents regardless.

Toxicity and overdose

Plumb's p.95: acute oral penicillin overdose is unlikely to cause more than GI upset. Very high PARENTERAL doses — particularly where renal function is impaired — have induced CNS effects in humans. The dominant real-world risk is not dose-related toxicity but (a) hypersensitivity/anaphylaxis and (b) fatal clostridial enterotoxaemia if given orally to rabbits, guinea pigs, chinchillas or hamsters.

Clinical pearls

• For oral therapy, amoxicillin is preferred — roughly twice the bioavailability of ampicillin
• Add a beta-lactamase inhibitor (sulbactam) or switch to amoxicillin-clavulanate for resistant Staph / E. coli
• Reconstituted sodium ampicillin is unstable — use within 1 h at room temperature or refrigerate and use within 8 h

Mechanism of action

Ampicillin is an aminopenicillin. Time-dependent, usually bactericidal — inhibits bacterial cell wall synthesis. Plumb's p.95: the aminopenicillins have increased activity against many gram-negative aerobes not covered by the natural or penicillinase-resistant penicillins (some E. coli, Klebsiella, Haemophilus), and retain useful anaerobic activity including Clostridium, though less than the natural penicillins. Susceptible to inactivation by beta-lactamase producers such as Staph. aureus — hence the sulbactam and clavulanate combinations. Generally NOT active against Pseudomonas aeruginosa, Serratia, indole-positive Proteus (P. mirabilis is susceptible), Enterobacter, Citrobacter or Acinetobacter, nor against Rickettsia, mycobacteria, fungi, Mycoplasma or viruses.

Formulations and products

• Ampicillin sodium 500 mg dry-powder vial — reconstitute per label (Papich 5e p.51)
• Ampicillin sodium 1 g / 2 g vial + 3.5 / 6.8 mL sterile water = 250 mg/mL (IM — Papich 5e p.51)
• Ampicillin trihydrate 250 mg capsule
• Ampicillin trihydrate 100 mg/mL suspension (Polyflex)

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

Other antibiotics

• Amikacin
• Amoxicillin
• Amoxicillin / Clavulanic Acid
• Apramycin
• Azithromycin
• Cefadroxil
• Cefazolin
• Cefixime

Frequently asked questions