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Dihydrofolate reductase inhibitor / antibiotic

Trimethoprim

Brand names: Trimethoprim, Monotrim

Trimethoprim is an antibiotic used for uncomplicated urinary tract infection and, at low dose, for UTI prophylaxis.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

US labelling (FDA)

Reference — US labelling, may differ from UK

DOSAGE AND ADMINISTRATION Sulfamethoxazole and trimethoprim tablets are contraindicated in pediatric patients less than 2 months of age. Urinary Tract Infections and Shigellosis in Adults and Pediatric Patients, and Acute Otitis Media in Children Adults The usual adult dosage in the treatment of urinary tract infections is 1 sulfamethoxazole and trimethoprim DS (double strength) tablet or 2 sulfamethoxazole and trimethoprim tablets every 12 hours for 10 to 14 days. An identical daily dosage is used for 5 days in the treatment of shigellosis. Children The recommended dose for children with urinary tract infections or acute otitis media is 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2024-12-17. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Clinical monograph

How it works

Trimethoprim inhibits bacterial dihydrofolate reductase, blocking folate synthesis and bacterial DNA production.

Prescribing in practice

  • Local resistance rates are significant — follow local sensitivities and guidance when choosing it empirically.
  • It can raise potassium and creatinine; use caution with ACE inhibitors, ARBs, potassium-sparing drugs and other potassium-raising agents.
  • Avoid in the first trimester of pregnancy (folate antagonism) and use caution with other folate antagonists such as methotrexate.

Monitoring

Short courses need no routine monitoring; consider renal function and potassium with longer use or in at-risk patients on interacting drugs.

Counselling the patient

  • Complete the prescribed course.
  • Report a rash, or symptoms that do not improve.

Evidence & guidelines

Trimethoprim is an option for uncomplicated lower UTI subject to local resistance patterns, per NICE/PHE antimicrobial guidance.

Reference: NICE NG112 UTI; PHE Antimicrobial Prescribing Guidelines; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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