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Trimethoprim /Sulfamethoxazole (Tribrissen®) for Dogs and Cats

Trimethoprim and sulfamethoxazole are two antibiotics, commonly called Tribrissen® or TMS, that work together to create an optimal effect. It is used in dogs and cats to treat pneumonia, prostatitis, kennel cough, otitis externa, and skin infections.

How Trimethoprim and Sulfamethoxazole Work

Trimethoprim’s method of action is to block the production of tetrahydrofolic acid from dihydrofolic acid by binding to and reversibly inhibiting the enzyme dihydrofolate reductase. This binding is much stronger for the bacterial enzyme than the corresponding mammalian enzyme.

Sulfamethoxazole inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid. Thus, the combination of trimethoprim and sulfamethoxazole inhibits two consecutive steps in the production of nucleic acids and proteins.

Trimethoprim and sulfamethoxazole are prescription drugs and can only be obtained from a veterinarian or by prescription from a veterinarian.

Brand Names of Trimethoprim and Sulfamethoxazole

This drug is registered for use in humans, dogs, horses, cattle, and swine.

Uses of Trimethoprim and Sulfamethoxazole for Dogs and Cats

Trimethoprim and sulfamethoxazole areused in both dogs and cats to treat a variety of infections including:

Potential Side Effects of Trimethoprim and Sulfamethoxazole

Trimethoprim and sulfamethoxazole should not be administered to pregnant animals or animals with known hypersensitivities or allergies to these or other sulfa drugs.
Trimethoprim and sulfamethoxazole should not be used in pregnant animals.

While generally safe and effective when prescribed by a veterinarian, trimethoprim and sulfamethoxazole can cause side effects in some animals. The following are known side effects organized by organ system:

Hematologic: Agranulocytosis, aplastic anemia, thrombocytopenia, leukopenia, neutropenia, hemolytic anemia, megaloblastic anemia, hypoprothrombinemia, methemoglobinemia, and eosinophilia.
Allergic Reactions: Epidermal necrolysis, anaphylaxis, allergic myocarditis, erythema multiforme, exfoliative dermatitis, angioedema, drug fever, purpura, serum sickness-like syndrome, generalized allergic reactions, generalized skin eruptions, photosensitivity, conjunctival and scleral injection, pruritus, urticaria, and rash. In addition, periarteritis nodosa and systemic lupus erythematosus have been reported in humans.
Gastrointestinal: Hepatitis, including cholestatic jaundice and hepatic necrosis, elevation of serum transaminases and bilirubin, pseudomembranous enterocolitis, pancreatitis, stomatitis, glossitis, nausea, emesis, abdominal pain, diarrhea, and anorexia.
Genitourinary: Renal failure, interstitial nephritis, BUN and serum creatinine elevation, toxic nephrosis with oliguria and anuria, and crystalluria.
Metabolic: Hyperkalemia, hyponatremia.
Neurologic: Aseptic meningitis, convulsions, peripheral neuritis, ataxia, vertigo, tinnitus, and headache.
Central Nervous System: Depression, apathy, and anxiety.
Endocrine: Cross-sensitivity may exist with diuretics (acetazolamide and thiazides), and oral hypoglycemic agents. Diuresis and hypoglycemia have occurred in human patients receiving sulfonamides. Trimethoprim-sulfamethoxazole combination has been demonstrated to reduce thyroid hormone levels to below the normal reference range.
Musculoskeletal: Arthralgia and myalgia.
Respiratory System: Cough, dyspnea, and pulmonary infiltrates.

Drug Interactions

Trimethoprim and sulfamethoxazole may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with trimethoprim and sulfamethoxazole.

When injectable preparations are mixed with other drugs, incompatibility may occur.
Sulfamethoxazole and trimethoprim may inhibit the hepatic metabolism of phenytoin.
Sulfonamides can displace methotrexate from plasma protein binding sites, increasing free methotrexate concentrations.

Other drug interactions occur when trimethoprim-sulfamethoxazole is coadministered with anticoagulants (prothrombin time increased), cyclosporine (decreased therapeutic efficacy of cyclosporine), and diuretics (increased incidence of thrombocytopenia with purpura in humans). Additionally, the hypoglycemic response to sulfonureas may be increased and serum levels of zidovudine may be increased.

How Trimethoprim & Sulphamethoxazole is Supplied

There are several human and veterinary preparations available.

Human Preparations


Oral Suspension:



Veterinary Preparations


Oral Paste (for horses):

Injection (for horses):

Trimethoprim/Sulfamethoxazole Dosage for Dogs and Cats

Doses of trimethoprim and sulfamethoxazole vary depending on the reason for prescribing.
The duration of administration depends on the condition being treated, response to the medication, and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.

In dogs and cats, the usual dose is 12.5 to 50 milligrams/pound of body weight (25 to 100 milligrams/kilograms) every 24 hours orally, subcutaneously, intramuscularly or intravenously for 10 to 20 days.

Medication should never be administered without first consulting your veterinarian. If you’re concerned about costs related to medication, click here to learn how pet insurance may be able to help.