# Ampicillin: veterinary dose and monograph

> Ampicillin is a veterinary antibiotic. Aminopenicillin, bactericidal, time-dependent. Indications include susceptible Gram-positive and Gram-negative infections; urinary, soft-tissue and respiratory infections; leptospirosis (with supportive care). This monograph lists 15 reference doses for dogs, cats, cattle, horses, pigs and birds and 6 more species, each with route, frequency and source, plus meat and milk withdrawal periods for food animals.

Source: https://vetmasterji.com/drugs/ampicillin · Last reviewed: 2026-10-01

## 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

*Ampicillin doses for dogs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Sepsis / severe systemic infection (sodium salt) | 20–40 mg/kg | IV | q6–8h | — |
| Susceptible infection (oral trihydrate) | 20 mg/kg | PO | q8h | — |

- 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

*Ampicillin doses for cats*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Susceptible infection | 10–20 mg/kg | SC | q8–12h | — |

- 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

*Ampicillin doses for cattle*

| Indication | Dose | Route | Frequency | Duration | Withdrawal |
| --- | --- | --- | --- | --- | --- |
| Susceptible infection (trihydrate, label) | 11 mg/kg | IM | q24h | not more than 7 days | 6d slaughter / 48h milk |
| Respiratory infection — high dose (extra-label) | 22 mg/kg | SC | q12h | — | 60d slaughter |

- 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

*Ampicillin doses for horses*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Susceptible infection (sodium salt) | 10–20 mg/kg | IV | q6–8h | — |

- 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

*Ampicillin doses for pigs*

| Indication | Dose | Route | Frequency | Duration | Withdrawal |
| --- | --- | --- | --- | --- | --- |
| Miniature/pet pig — susceptible infection | 10–20 mg/kg | IM | q6–8h | — | Withdrawal NOT established — treat as extra-label |
| Susceptible infection (trihydrate) | 15 mg/kg | IM | q24h | — | Withdrawal NOT established — treat as extra-label |

- 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.

*Ampicillin doses for buffalo*

| Indication | Dose | Route | Frequency | Duration | Withdrawal |
| --- | --- | --- | --- | --- | --- |
| Susceptible infection (trihydrate, label) | 11 mg/kg | IM | q24h | not more than 7 days | 6d slaughter / 48h milk |
| Respiratory infection — high dose (extra-label) | 22 mg/kg | SC | q12h | — | 60d slaughter |

- 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

*Ampicillin doses for birds*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Susceptible infection | 100 mg/kg | IM | q4–8h | — |

- 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

*Ampicillin doses for rabbits*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Contraindicated | Do not use in rabbits — can cause fatal enteritis | — | — | — |

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

### Ferrets

*Ampicillin doses for ferrets*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Susceptible infection | 5–10 mg/kg | IM | q12h | — |

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

### Guinea pigs

*Ampicillin doses for guinea pigs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Contraindicated | Do not use in guinea pigs — may cause fatal enterocolitis | — | — | — |

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

### Reptiles

*Ampicillin doses for reptiles*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Chelonians — susceptible infection | 50 mg/kg | IM | q12h | — |

- 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

*Ampicillin doses for chinchillas*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Contraindicated | Do not use in chinchillas — may cause fatal enterocolitis | — | — | — |

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

### Hamsters

*Ampicillin doses for hamsters*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Contraindicated | Do not use in hamsters — may cause fatal enterocolitis | — | — | — |

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

### Rats

*Ampicillin doses for rats*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Susceptible infection | 20–100 mg/kg | SC | q8–12h | — |

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

### Mice

*Ampicillin doses for mice*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Susceptible infection | 20–100 mg/kg | SC | q8–12h | — |

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

### Hedgehogs

*Ampicillin doses for hedgehogs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Susceptible infection | 10 mg/kg | IM | q24h | — |

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

### Primates

*Ampicillin doses for primates*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Susceptible infection (prosimians, lemurs) | 10–30 mg/kg | IM | q6–8h | — |

- 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

> **Caution:** Rabbits: NEVER orally — fatal enterotoxaemia.

> **Caution:** 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

> **Caution:** 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](https://vetmasterji.com/drugs/amikacin)
- [Amoxicillin](https://vetmasterji.com/drugs/amoxicillin)
- [Amoxicillin / Clavulanic Acid](https://vetmasterji.com/drugs/amoxicillin-clavulanate)
- [Apramycin](https://vetmasterji.com/drugs/apramycin)
- [Azithromycin](https://vetmasterji.com/drugs/azithromycin)
- [Cefadroxil](https://vetmasterji.com/drugs/cefadroxil)
- [Cefazolin](https://vetmasterji.com/drugs/cefazolin)
- [Cefixime](https://vetmasterji.com/drugs/cefixime)

## Frequently asked questions

### What is the dose of Ampicillin for dogs?

Reference doses for Ampicillin in dogs: 20–40 mg/kg IV q6–8h (Sepsis / severe systemic infection (sodium salt)); 20 mg/kg PO q8h (Susceptible infection (oral trihydrate)). Confirm against the label and the patient before use.

### What is the dose of Ampicillin for cats?

Reference doses for Ampicillin in cats: 10–20 mg/kg SC q8–12h (Susceptible infection). Confirm against the label and the patient before use.

### What is the dose of Ampicillin for cattle?

Reference doses for Ampicillin in cattle: 11 mg/kg IM q24h for not more than 7 days (Susceptible infection (trihydrate, label)); 22 mg/kg SC q12h (Respiratory infection — high dose (extra-label)). Confirm against the label and the patient before use.

### What is the withdrawal period for Ampicillin?

Cattle, IM (Susceptible infection (trihydrate, label)): 6d slaughter / 48h milk; Cattle, SC (Respiratory infection — high dose (extra-label)): 60d slaughter; Pigs, IM (Miniature/pet pig — susceptible infection): Withdrawal NOT established — treat as extra-label; Pigs, IM (Susceptible infection (trihydrate)): Withdrawal NOT established — treat as extra-label. Follow the product label and national regulations where they differ.

### When should Ampicillin not be used?

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

### What are the side effects of Ampicillin in animals?

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

## Sources

- Plumb's Veterinary Drug Handbook
- Papich, Papich Handbook of Veterinary Drugs, 5th edition
- Carpenter, Exotic Animal Formulary

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VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.