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Sulfamethoxazole/Trimethoprim
What is sulfamethoxazole/trimethoprim?
Sulfamethoxazole/trimethoprim (brand names: Co-trimoxazole®, Primsol®, Bactrim®, Sulfatrim®, Novo-Trimel®, Septra®) is a potentiated sulfonamide antibiotic/antimicrobial used to treat certain infections such as bladder and prostate infections, Nocardia infections, or parasitic infections.
Its use in cats, dogs, birds, reptiles, and small mammals to treat infections is “off label” or “extra label”. Many drugs are commonly prescribed for off-label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully, as their directions may be significantly different from those on the label.
How is sulfamethoxazole/trimethoprim given?
This medication will take effect quickly, in about 1 to 2 hours, but visible effects may take a few days.
People with allergies to sulfa compounds should avoid handling this medication or should use gloves while administering this medication.
If you miss a dose and the next scheduled dose is 6 or more hours away, give the missed dose when you remember and then continue with the regular dosing schedule. However, if the next scheduled dose is less than 6 hours away, give the missed dose, skip the next scheduled dose, and then return to the regular dosing schedule. Never give your pet two doses at once or give extra doses. For example:
In all species, an increased risk of urinary crystals, blood in the urine, and urinary obstruction are possible. In rabbits, low tear production/dry eye is possible.
Common side effects in dogs include:
Possible serious side effects in dogs include:
Less common side effects in dogs include:
There is suspicion it may cause pancreatic inflammation in dogs, but this has not been definitively shown.
Common side effects in cats include:
More severe side effects in cats include:
This short-acting medication should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.
Sulfamethoxazole/trimethoprim SHOULD NOT be used in:
Sulfamethoxazole/trimethoprim should be used cautiously in:
Certain medications should be used with caution when given with sulfamethoxazole/trimethoprim, including but not limited to:
Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking before starting any treatment.
If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility.
© Copyright 2025 LifeLearn Inc. Used and/or modified with permission under license. This content written by LifeLearn Animal Health (LifeLearn Inc.) is licensed to this practice for the personal use of our clients. Any copying, printing or further distribution is prohibited without the express written consent of LifeLearn. This content does not contain all available information for any referenced medications and has not been reviewed by the FDA Center for Veterinary Medicine, or Health Canada Veterinary Drugs Directorate. This content may help answer commonly asked questions, but is not a substitute for medical advice, or a proper consultation and/or clinical examination of your pet by a veterinarian. Please contact your veterinarian if you have any questions or concerns about your pet’s health. Last updated on May 21, 2019.
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