By Lauren Forsythe, PharmD, DICVP; Rania Gollakner, BS, DVM, MPH

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?

  • Give sulfamethoxazole/trimethoprim by mouth, in the form of a tablet or liquid suspension or solution. 
  • Give with or without food.
  • If your pet vomits or acts sick after receiving the medication on an empty stomach, give the next dose with a meal or treat.
  • The liquid suspension should be shaken well before use and does not need to be refrigerated.
  • There is also an injectable form that is used in exotic pets and given in the muscle or under the skin.
  • Allow free choice access to water, as pets should not be allowed to become dehydrated while on this medication.

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.

What if I miss giving my pet the 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:

  • If you are dosing at 8 a.m. and 8 p.m., but you forget the 8 a.m. dose and remember at 11 a.m., give the dose at 11 a.m., then again at 8 p.m., and again at 8 a.m. the following morning. 
  • If you miss the 8 a.m. dose and remember at 3 p.m., give it at 3 p.m., but skip the 8 p.m. dose, and give it at 8 a.m. the following morning.

Are there any potential side effects?

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:

  • decreased appetite
  • vomiting
  • diarrhea

Possible serious side effects in dogs include:

  • keratoconjunctivitis sicca (dry eye)
  • acute liver inflammation characterized by yellowing of the skin/eyes
  • fever
  • hemolytic anemia
  • urticaria (hives)
  • polyarthritis
  • facial swelling
  • increased drinking and urination 
  • cholestasis (gall bladder attack)

Less common side effects in dogs include:

  • inducing hypothyroidism (low thyroid levels)
  • allergic reactions including anaphylaxis and immune reactions
  • anemia (low red blood cells)
  • low white blood cells
  • severe and massive liver damage (rarely) 

There is suspicion it may cause pancreatic inflammation in dogs, but this has not been definitively shown.

Common side effects in cats include:

  • decreased appetite 
  • increased salivation

More severe side effects in cats include:

  • low white blood cells 
  • low red blood cells

This short-acting medication should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

Sulfamethoxazole/trimethoprim SHOULD NOT be used in:

  • dogs with liver tissue damage
  • dogs with blood cell problems
  • dogs that are dehydrated 
  • dogs at an increased risk for urinary stones
  • pets allergic to sulfonamides
  • pets near the end of their pregnancy

Sulfamethoxazole/trimethoprim should be used cautiously in:

  • pets with liver or kidney disease
  • pets that are old, sick, or debilitated
  • pets with a folate deficiency
  • pets that are pregnant or nursing
  • doberman pinschers, as they may be more susceptible to adverse side effects

Are there any drug interactions I should be aware of?

Certain medications should be used with caution when given with sulfamethoxazole/trimethoprim, including but not limited to:

  • amantadine
  • antacids
  • cyclosporine
  • potassium supplements

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking before starting any treatment.

Is there any monitoring that needs to be done with this medication?

  • Your pet should be monitored for adverse side effects.
  • For prolonged therapy, your veterinarian will recommend baseline and ongoing complete blood counts and tear tests. 
  • A baseline and ongoing thyroid function test may also need to be monitored. 
  • Your veterinarian may monitor your pet to be sure that the medication is working.

How do I store sulfamethoxazole/trimethoprim?

  • Follow the storage directions as instructed by the manufacturer. 
  • Store all products at room temperature at 20°-25°C (68°-77°F). 
  • Do not allow the medication to freeze 
  • Keep in a tightly sealed container.

What should I do in case of emergency?

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.

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