Antibiotics Dogs & Cats

Trimethoprim-sulfa for dogs and cats

Treats bladder and prostate infections and some parasitic infections. [1]

Last reviewed: October 2026 · 2 sources · see sources

Quick answerTrimethoprim-sulfa (sulfamethoxazole/trimethoprim) is a prescription medicine for dogs and cats. Two drugs that block two steps in the way bacteria make folate, which they need to grow. The most common side effects: low appetite, vomiting and diarrhea.

Key facts

Generic nameSulfamethoxazole/trimethoprim
Common brand namesBactrim, Septra, Primsol
Used inDogs & Cats
TypeAntibiotics
Prescription?Prescription
Typical US priceNot listed (no reliable public source)

What it's used for

Treats bladder and prostate infections and some parasitic infections. [1]

How it works

Two drugs that block two steps in the way bacteria make folate, which they need to grow. [1]

Side effects and cautions

Common side effects: Low appetite, vomiting and diarrhea. More serious: dry eye, liver inflammation, fever, hives and joint pain. Urinary crystals are possible. [1]

Use with care: Your vet may check tear production during long courses. Doberman pinschers may be more prone to reactions to sulfa drugs. [1]

Call your vet right away if your pet has trouble breathing, facial swelling, hives, collapse, or any side effect that is severe or getting worse.

About dosingDosing is your vet's call. We don't list doses because the right amount depends on your pet's weight, age, health and other medicines. Don't change a dose or stop a medicine without asking your vet, and never give a pet medicine meant for people unless your vet tells you to [2].

What it costs

We don't list a price for this one because we couldn't find a reliable public source. Ask your vet or pharmacy for a quote; prices depend on your pet's size, the dose and where you fill it.

Trimethoprim-sulfa FAQ

What is Trimethoprim-sulfa used for in dogs and cats?

Treats bladder and prostate infections and some parasitic infections.

What are the side effects of Trimethoprim-sulfa?

Low appetite, vomiting and diarrhea. More serious: dry eye, liver inflammation, fever, hives and joint pain. Urinary crystals are possible.

Does Trimethoprim-sulfa need a prescription?

Yes. It needs a prescription from a veterinarian.

How much Trimethoprim-sulfa should I give my pet?

Only your veterinarian can tell you. The right dose depends on your pet's weight, health and other medicines, so we don't list doses. Never give a pet a medicine prescribed for a person or another animal without asking your vet.

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This page is general information for pet owners, not veterinary advice. Talk to your vet about your own pet.

Sources (2) — tap to show
  1. VCA Animal Hospitals – Sulfamethoxazole/trimethoprim (veterinary drug handout)
  2. FDA – Frequently asked questions about animal drugs