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Trimethoprim Sulfa

Antimicrobial Agents
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(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)


5 mg trimethoprim/25 mg sulfadiazine
20 mg trimethoprim/100 mg sulfadiazine
80 mg trimethoprim/400 mg sulfadiazine
160 mg trimethoprim/800 mg sulfadiazine (double strength tablet)

40 mg trimethoprim/200 mg sulfamethoxazole
80 mg trimethoprim/400 mg sulfamethoxazole
160 mg trimethoprim/800 mg sulfamethoxazole (double strength tablet)
240 mg/5 mL (containing 40 mg trimethoprim and 200 mg sulfamethoxazole) suspension


Trimethoprim / sulfa is a combination antimicrobrial 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. However, it does not appear to be effective against either mycoplasma, or Pseudomonas aeruginosa. The combination of trimethoprim / sulfa also appears to inhibit some protozoa (e.g., Pneumocystis carinii).

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.

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 rats that are nursing.


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 drug.

Adverse Reactions

Use with caution in preexisting hepatic disease.

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

Note: 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.

Trimethoprim-sulfamethoxazole: 15 mg/kg to 30 mg/kg, PO, q12hrs to q24hrs  2


15 mg/kg to 30 mg/kg, PO,IM, q12hrs  27


15 mg/kg, q12hrs, for 10-21 days  18


30 mg/kg, q12hrs, for 7-10 days 30


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


30 mg/kg, PO, SQ (*note: tissue necrosis may occur when given SQ), IM, q12hrs  1


48 mg/kg to 96 mg/kg, PO, q24hr  34


Trimethoprim-sulfadiazine: 30 mg/kg, SQ (*note: tissue necrosis may occur when given SQ), q12hrs to q24hrs  2

*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.


  • Terrible tasting give with a bit of syrup.
  • Keep rat well hydrated while on medication.
  • When using the suspension be sure to shake well. Suspension does not require refrigeration.
  • 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).  1
  • 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.

Posted on June 23, 2003, 14:38, Last updated on September 10, 2016, 14:12 | Antimicrobial Agents

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