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?

Sulfamethoxazole/trimethoprim is given by mouth with or without food in the form of a tablet or liquid suspension or solution. 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 to be observed.

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 it is not within 6 hours of the next scheduled dose, give the missed dose when you remember and then continue with the regular dosing schedule. If you miss a dose and it is within 6 hours of the next dose, give the missed dose, skip the dose when you remember, skip the next dose, and then return to the regular dosing schedule. Never give your pet two doses at once or give extra doses.

Example: If dosing at 8AM and 8PM, and you forget the 8AM dose and remember at 11AM, give it at 11AM, then again at 8PM, then at 8AM the following morning. If you forget the 8AM dose and remember at 3PM, give it at 3PM, skip the 8PM dose, and give it at 8AM the following morning.


Are there any potential side effects?

Common side effects in dogs include a decreased appetite, vomiting, and diarrhea. Possible serious side effects 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, and cholestasis (gall bladder attack).

Less common side effects include inducing hypothyroidism (low thyroid levels), allergic reactions including anaphylaxis and immune reactions, anemia (low red blood cells) and low white blood cells. Rarely, it can cause severe and massive liver damage. There is suspicion it may cause pancreatic inflammation, but this has not been definitively shown.

In cats, a decreased appetite or increased salivation is common. More severe side effects include low white blood cells and low red blood cells. In rabbits, low tear production/dry eye is possible.

In all species, an increased risk of urinary crystals, blood in the urine, and urinary obstruction are possible.

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, blood cell problems, dogs that are dehydrated or at an increased risk for urinary stones, or those allergic to sulfonamides. It should be used cautiously in Doberman Pinschers as they may be more susceptible to adverse side effects. It should be used with caution in pets with liver or kidney disease; old, sick, or debilitated pet; or pets with a folate deficiency. Sulfamethoxazole/trimethoprim should be used cautiously in pregnant and nursing pets, and should not be used in are near the end of their pregnancy.


Are there any drug interactions I should be aware of?

The following medications should be used with caution when given with sulfamethoxazole/trimethoprim: amantadine, antacids, cyclosporine, digoxin, thiazide diuretics, oral hypoglycemic agents, methotrexate, phenytoin, tricyclic antidepressants, warfarin, potassium supplements, potassium-sparing drugs, and pyrimethamine. 

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


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. Otherwise, store all products at room temperature at 20°-25°C (68°-77°F). Do not allow the medication to freeze and 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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