Trimethoprim Sulfa

(trimethoprim sulfadiazine,  trimethoprim sulfamethoxazole
synonymous with:   Cotrimoxazole, SMX-TMP, TMP-SMX, TMP-SDZ, SDZ-TMP, Co-trimazine)


Tribrissen (trimthoprim-sulfadiazine), Bactrim, Bactrim-DS, Septra, Septra DS, Cotrim, Cotrim-DS (trimethoprim-sulfamethoxazole)


    Tablets: trimethoprim 5 mg/sulfadiazine 25 mg

  • Tablets: trimethoprim 20 mg/sulfadiazine 100 mg
  • Tablets: trimethoprim 80 mg/sulfadiazine 400 mg
  • Double strength tablets: trimethoprim 160 mg/sulfadiazine 800 mg
  • Tablets: trimethoprim 40 mg/sulfamethoxazole 200 mg
  • Tablets: trimethoprim 80 mg/sulfamethoxazole 400 mg
  • Double strength tablets: trimethoprim 160 mg/sulfamethoxazole 800 mg
  • Suspension: 240 mg/5mL (containing trimethoprim 40 mg and sulfamethoxazole 200 mg)

*Note: tablets, or injectable may be used to make a suspension.


Trimethoprim / sulfa is a combination antimicrobial drug, which by two consecutive steps blocks protein synthesis in susceptible bacteria.
When used alone the sulfonamides are bacteriostatic, and the trimethoprim is bactericidal, but when used in combination form the potentiated sulfas are bactericidal.

This combination drug has broad-spectrum activity. It is effective against Gram-positive bacteria such as streptococci and some strains of staphylococcus. Gram-negative organisms such as CAR bacillus and those of the Enterobacteriaceae are also susceptible to this drug. The combination of trimethoprim / sulfa also appears to inhibit some protozoa (e.g., Pneumocystis carinii). However, the drug does not appear to be very effective against either mycoplasma, or Pseudomonas aeruginosa.

Trimethoprim / sulfa is able to penetrate into exudate and abscessed tissue, as well as penetrate the blood-brain-eye-placenta barrier. It is well absorbed orally with minimal effect on flora of the GI tract.

The safety of trimethoprim / sulfa during pregnancy is not clearly established. Reports indicate teratogenicity (cleft palate) in studies with rats. Therefore, it is recommended that it not be given to pregnant rats, unless benefit outweighs risk.

Trimethoprim / sulfa is metabolized by the liver and excreted during glomerular filtration and tubular secretion. Because sulfonamides are found to be excreted in breast milk it is advised to avoid giving trimethoprim / sulfa in the female rat that may be nursing young.


Good choice for use in urinary tract infections or secondary infections.

Drug Interactions or Contraindications

  • Use enhances NSAIDs, thiazide diuretics (as in hydrochlorothiazide), and aspirin.
  • Antacids may interfere with efficacy of this drug.
  • Use with caution in preexisting hepatic disease.

Adverse Reactions

Blood: anemias, dyscrasias

GI: diarrhea

Renal: crystalluria (sulfa bladder stones)

Skin: rash/itching

Skeletal:  joint inflammation

Other: hypersensitivity/anaphylaxis, interference in tear production

Dosage Recommendations

Dosages are for Trimethoprim plus sulfonamide combinations.

Note:The dosages are for Trimethroprim plus sulfonamide combinations. Dosing is based on the trimethoprim component unless otherwise stated. Due to the variety of tablet strengths, where the commercial suspension of 240mg/5mL is not used, it is important to have the veterinarian calculate the correct dose to give.

15 mg/kg to 30 mg/kg, PO, q12hr to q24hr  2


15 mg/kg to 30 mg/kg, PO, IM, q12hr  27


15 mg/kg, q12hr, for 10 to 21 days  18


15 mg/kg to 30 mg/kg, PO, SQ (tissue necrosis may occur when given SQ), IM q12hr to q24hr  35, 41, 44


25 mg/kg, PO, q12hr  41, 44


30 mg/kg, q12hr, for 7-10 days 30


30 mg/kg, SQ (tissue necrosis may occur when given SQ), q12hr to q24hr  2


15 mg/lb, PO, BID (as recommended on RMCA Drug Chart4, 12


30 mg/kg, PO, SQ (tissue necrosis may occur when given SQ), IM, q12hr  1, 26


50 mg to 100 mg, PO, SQ (tissue necrosis may occur when given SQ), q24hr  41, 42, 44

*Note: the initials DS stand for double strength on some of the brand names. One may see the vet prescribe this at a different frequency.


  • The medicine has a bitter terrible taste give with a bit of syrup.
  • Keep rat well hydrated while on medication.
  • This medication is best not used in pregnancy. Birth defects have been reported after this medication was given to pregnant rats.
  • In treating suspected polymicrobial infections, where a broader coverage may be needed, synergistic or combination drugs may be used. The following drugs may be seen used simultaneously with trimethoprim sulfa: aminoglycosides (e.g., amikacin or gentamicin).
  • Please note that it is imperative to discuss the changing or adding of any medications during your rat’s treatment with your veterinarian to prevent future resistance of microbes to the drugs prescribed.
  • When using the suspension be sure to shake well. Suspension does not require refrigeration.


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