Selected Antimicrobial Information – Companion Animal

Ampicillin [Companion]

Restriction Status



Species Usage Dose

Label dose 6.6mg/kg SQ or IM q12h
For systemic infections
10-20mg/kg IV, IM, or SQ q8-12h (extra-label use)

Label dose 6.6mg/kg SQ or IM q12h
For soft tissue infections 10-20mg/kg SQ or IM q12h for 7d (extra-label use)
For sepsis, meningitis, or orthopedic infections 22mg/kg IV, IM, or SQ q6-8h (extra-label use)
For neonatal sepsis
50mg/kg IV or IO q4-6h (extra-label use)
For leptospirosis 20mg/kg IV q6h until de-escalation to oral therapy is possible (extra-label use)
For infectious endocarditis 20-40mg/kg IV q6-8h

Brand Name(s)



Ampicillin is a bactericidal, time-dependent β-lactam antibiotic with improved activity against Gram-negative organisms when compared to penicillin, though with somewhat less activity against Gram-positive organisms. There is also some efficacy against anaerobes. Ampicillin distributes widely through the plasma, and can cross into the prostate, the eye, and the blood-brain barrier when there is inflammation. It does not penetrate abscesses or sites of tissue necrosis well. Excretion is primarily as active and metabolized drug in urine.

Except for uncomplicated (lower) urinary tract infections, ampicillin should not be used to treat infections caused by Enterobacterales (e.g., E. coli, Klebsiella spp., Proteus spp., Enterobacter spp.) due to the inability of this drug to reach an effective inhibitory serum concentration for the required period when recommended dose regimen are used.

Acceptable Uses

  • Treatment of bacterial respiratory tract infections.
  • Treatment of soft tissue or orthopedic infections.
  • Treatment of gastrointestinal infections associated with susceptible Enterococcus spp., Staphylococcus spp., or E. coli.
  • Treatment of bacterial endocarditis, bacteremia, or sepsis. Often used in combination with an aminoglycoside or a fluoroquinolone.
  • Initial treatment for leptospirosis in dogs who cannot tolerate oral doxycycline administration due to intractable vomiting.
  • As a parenteral alternative to amoxicillin.
  • Treatment of susceptible soft tissue infections (except for infections caused by Enterobacterales and Staphylococcus spp).

Unacceptable Uses

  • When used alone, empirical treatment of suspected Staphylococcus spp. infections due to widespread resistance.
  • Soft tissue infections (e.g., wounds, otitis, pneumonia, among others) caused by Enterobacterales and Staphylococcus spp.

Formulations Available within the OSU Pharmacy

  • Ampicillin 1g/vial injectable suspension
  • Ampicillin 500mg/vial injectable suspension
  • Ampicillin NA 1g/vial injectable suspension
  • Ampicillin NA 500mg/vial injectable suspension


  • Ampicillin trihydrate (Polyflex®) should not be given IV due to risk of anaphylaxis and sudden death. Ampicillin sodium is safe for IV injection.
  • Although ampicillin is recommended as initial treatment in patients with leptospirosis that will not tolerate oral medications (i.e. because of vomiting), treatment with ampicillin does not clear infection with Leptospira spp. in companion animals and should be followed by oral doxycycline once vomiting resolves.


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OSU VMC Antimicrobial Use Guidelines Copyright © 2018 by The Ohio State University College of Veterinary Medicine is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.