Selected Antimicrobial Information – Companion Animal

Trimethoprim- or Ormetoprim-Sulfonamide Combinations [Companion]

Restriction Status



Ormetoprim-Sulfadimethoxine (Primor®)

Species Usage Dose
Cats Not Applicable N/A
Dogs Label dose 55mg/kg PO once followed by 27.5mg/kg PO q24h for no longer than 21d

Trimethoprim-Sulfadiazine or Trimethoprim-Sulfamethoxazole (Tribrissen®)

Species Usage Dose
Cats For soft tissue infections and uncomplicated UTIs 15mg/kg PO q12h for 7-14d (extra-label use)
Dogs Label dose 30mg/kg PO q24h or 15mg/kg PO q12h
For systemic infections or bacteremia 30-45mg/kg PO or SQ q12h for 3-5d (extra-label use)

Brand Name(s)

Tribrissen®, Primor®, etc.


Trimethoprim- or ormetoprim-sulfonamides are a group of time-dependent potentiated sulfonamide antibiotics. Sulfonamides are bacteriostatic when used alone, but bactericidal when used in combination with trimethoprim or ormetoprim. They are broadly effective against many Gram-positive and Gram-negative aerobic bacteria, but are not effective against Pseudomonas spp., Mycoplasma spp., and most isolates of Klebsiella spp. In vivo activity against anaerobes is poor. Distribution is wide, and therapeutic concentrations can be achieved intracellularly and across the blood-brain barrier. Excretion is primarily as unchanged drug in urine.

Acceptable Uses

  • Treatment of many systemic infections including soft tissue and skin infections, pyoderma, urinary tract infections, respiratory tract infections, gastrointestinal infections, etc.

Unacceptable Uses

  • Should never be used in black-and-tan dog breeds.

Formulations Available within the OSU Pharmacy

  • Sulfamethoxazole/trimethoprim 400/80 tablet
  • Sulfamethoxazole/trimethoprim 800/160 tablet
  • Primor 600mg tablet


  • Immune-mediated adverse effects such as keratoconjunctivitis sicca, immune-mediated thrombocytopenia, or immune-mediated hemolytic anemia may occur in dogs. Immune-mediated polyarthropathy has been noted in black-and-tan dogs and should therefore not be used in these breeds or mixed-breeds. Obtaining a baseline Schirmer tear test value and weekly monitoring thereafter are recommended to determine if adverse effects are developing. Serial CBC monitoring may also be useful for patients receiving prolonged treatment.
  • Leukopenia and anemia are also adverse effects in cats.


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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.