Skip to content
ClinCalc Pro
Menu
Calculators Drugs Pathways AI Dx Donate Get the App
Antifolate Antibiotic — Urinary (Obstetric Caution) Pregnancy: Contraindicated in pregnant women and in the first trimester of pregnancy (teratogenic in animal studies; increased risk of spontaneous abortion and congenital malformations - neural tube defects, oral clefts, cardiovascular defects - after first-trimester exposure). In the second and third trimesters use should be avoided unless clinically necessary. Excreted in breast milk; contraindicated if the breast-fed infant is less than 4 months of age.

Trimethoprim (UTI in Pregnancy — Caution)

Brand names: Monotrim

Trimethoprim is a folate-antagonist antibacterial commonly used for urinary tract infections.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 200 mg (20 ml) twice daily
Route: Oral
Frequency: Twice daily
PREGNANCY (this page's context): the SPC states trimethoprim should NOT be administered to pregnant women and is CONTRAINDICATED during the first trimester (folate antagonist; epidemiological studies show increased risk of spontaneous abortion and congenital malformations, in particular neural tube defects, oral clefts and cardiovascular defects). In the second and third trimesters, use should be avoided unless clinically necessary. Contraindicated in breast-feeding if the infant is under 4 months of age. DOSING (eMC 4.2, Trimethoprim 50 mg/5 ml Suspension): adults and children over 12 years - treatment of urinary tract infections and all other susceptible infections 200 mg (20 ml) twice daily; long-term prevention of recurrent urinary tract infections 100 mg (10 ml) at night. Elderly: dose depends on kidney function (see renal schedule). Monitoring of renal function and serum electrolytes should be considered particularly with longer-term use in patients with impaired renal function. Trimethoprim is removed by dialysis; it should only be initiated and used in dialysis patients under close supervision from specialists in both infectious disease and renal medicine. Regular haematological tests should be performed during long-term therapy.

Paediatric dose

Dose: 8 mg/kg
Route: Oral
Frequency: Daily, subdivided into two equal doses (children 6 weeks to 12 years; treatment of urinary tract infection)
Treatment of UTI: 8 mg/kg body weight daily, subdivided into two equal doses. Suggested regimens: 6 weeks-5 months 25 mg (2.5 ml) twice daily; 6 months-5 years 50 mg (5 ml) twice daily; 6-12 years 100 mg (10 ml) twice daily. Long-term prevention of recurrent UTI: 2 mg/kg body weight daily as a single dose at night (6 months-5 years 25 mg/2.5 ml at night; 6-12 years 50 mg/5 ml at night). Contraindicated in premature infants and neonates under 6 weeks. Particular care is needed with haematological monitoring of children on long-term therapy. Verify paediatric dosing against a current children's formulary.

Dose adjustments

Renal

eGFR over 30 ml/min: normal dose. eGFR 15-30 ml/min: normal dose for 3 days then half dose. eGFR under 15 ml/min: half the normal dose. Monitor renal function and serum electrolytes, particularly with longer-term use.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Paediatric weight-based calculator

Treatment of UTI: 8 mg/kg body weight daily, subdivided into two equal doses. Suggested regimens: 6 weeks-5 months 25 mg (2.5 ml) twice daily; 6 months-5 years 50 mg (5 ml) twice daily; 6-12 years 100 mg (10 ml) twice daily. Long-term prevention of recurrent UTI: 2 mg/kg body weight daily as a single dose at night (6 months-5 years 25 mg/2.5 ml at night; 6-12 years 50 mg/5 ml at night). Contraindicated in premature infants and neonates under 6 weeks. Particular care is needed with haematological monitoring of children on long-term therapy. Verify paediatric dosing against a current children's formulary.

Verify in a children's formulary

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.

Contraindications

  • Hypersensitivity to the active substance or to any of the excipients
  • Severe hepatic insufficiency
  • Megaloblastic anaemia and other blood dyscrasias
  • Pregnancy (first trimester)
  • Premature infants and neonates under 6 weeks

Side effects

  • Hyperkalaemia (very common)
  • Nausea, diarrhoea, vomiting (common)
  • Headache (common)
  • Skin rashes, urticaria; monilial overgrowth (common)
  • Very rare/serious: bone marrow depression and blood dyscrasias (leucopenia, neutropenia, thrombocytopenia, agranulocytosis, aplastic anaemia); severe cutaneous adverse reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS)

Interactions

  • Potassium-sparing diuretics, potassium supplements, potassium-containing salt substitutes, renin-angiotensin system inhibitors (ACE inhibitors / ARBs) and other drugs associated with raised potassium (e.g. heparin): risk of hyperkalaemia - monitor serum potassium
  • Repaglinide: monitoring of blood glucose is advised on co-administration

Clinical monograph

How it works

It inhibits bacterial dihydrofolate reductase, blocking folate synthesis and so impairing bacterial DNA production.

Prescribing in practice

  • Avoid in the first trimester of pregnancy — as an antifolate it is associated with an increased risk of neural tube defects.
  • It can cause hyperkalaemia and a reversible rise in serum creatinine; use caution with ACE inhibitors, angiotensin receptor blockers and potassium-sparing drugs.
  • It must not be combined with methotrexate, as the additive antifolate effect can precipitate severe bone-marrow suppression.

Monitoring

For short courses in healthy patients little monitoring is needed, but check renal function and potassium in those at risk (renal impairment, older patients, or on interacting drugs such as ACE inhibitors).

Counselling the patient

  • Tell your prescriber if you are or might be pregnant, as it is avoided in early pregnancy.
  • Complete the prescribed course and report a rash, sore throat or unusual bruising.
  • Mention any methotrexate or blood-pressure or potassium medicines you take, as these interact.

Evidence & guidelines

An established option for urinary tract infection where local resistance patterns allow; UK guidance directs first-trimester avoidance and folate-supplement consideration, and choice should reflect local sensitivity data.

Reference: NICE NG109 (UTI in Adults); NICE NG25 (Antenatal Care); RCOG Antenatal Care; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

Calculators