Co-trimoxazole (Sulfamethoxazole/Trimethoprim)

C. diff Risk


Oral Bioavailability


Approximate Cost

IV$100/d PO$1/d


800 mg/160 mg (1 DS) tablet PO q12h

Use caution in infants <2 months old due to risk of kernicterus.

Renally cleared, requires dosage adjustment with changes in renal function. Consult a pharmacist for renal dosing.

General Information

Common Usage

  •  Urinary tract infections

  •  Susceptible MRSA infections

  •  Stenotrophomonas infections

  •  Pneumocystis jirovecii pneumonia (treatment or prophylaxis)

Drug Monitoring

  •  Creatinine and electrolytes in patients at increased risk renal failure

  •  Potassium

  •  CBC

Adverse Effects

  •  Stevens Johnson syndrome/toxic epidermal necrolysis & other rashes

  •  Gastrointestinal upset (common)

  •  Bone marrow suppression

  •  Hyperkalemia

  •  Renal failure

  •  Hepatitis

  •  Aseptic meningitis

Major Interactions

  •  Methotrexate- marrow suppression

  •  Increases phenytoin

  •  Increases INR with warfarin

Oral Liquids

Concentration: (per 1 mL) Trimethoprim 8 mg and Sulfamethoxazole 40 mg
Taste: very palatable
Not all strengths of oral liquids are listed nor are available on IWK formulary

Tablet/Capsule Strengths

Pediatric Tablet: Trimethoprim 20 mg and Sulfamethoxazole 100 mg

Adult Tablet: Trimethoprim 80 mg and Sulfamethoxazole 400 mg

Double Strength Tablet: Trimethoprim 160 mg and Sulfamethoxazole 800 mg

Tablets and capsules are preferred especially over an unpleasant tasting liquid.
Not all strengths of oral tablets/capsules are listed and they are not all available on the IWK formulary.

Additional Information

Dosing is based on the trimethoprim (TMP) component

Pediatric strength tablets:100mg SMX/ 20 mg TMP
Regular strength tablets: 400mg SMX/ 80 mg TMP
Double strength tablets: 800 mg SMX/ 160 mg TMP
Suspension: (per 1 mL): 40 mg SMX/8 mg TMP
Injection: (per 1 mL): 80 mg SMX/16 mg TMP


Antimicrobial class: Sulfonamide - Antifolate

Average serum half life: TMP (prolonged in renal failure)

  • Newborns: ~19 hours; range: 11-27 hours

  •  Infants 2 months to 1 year: ~4.6 hours; range: 3-6 hours

  •  Children 1-10 years: 3.7-5.5 hours

  •  Children and Adolescents >10 years: 8.19 hours

  •  Adults: 6-11 hours

SMX: 9-12 hours, prolonged in renal failure (Adult data)

Route of Elimination: Both excreted in urine as metabolites and unchanged drug