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Antibiotic — Folate Synthesis Inhibitor Pregnancy: Trimethoprim is contra-indicated in pregnant women, premature infants or infants during the first few weeks of life. Trimethoprim is contraindicated during the first trimester of pregnancy (see section 4.3). Studies in animals have shown a teratogenic effect. Epidemiological studies have shown an increased risk of spontaneous abortion and congenital malformations, in particular neural tube defects, oral clefts and cardiovascular defects, in children of mothers treated with trimethoprim during the first trimester of pregnancy. The presumed mechanism of action is thought to be interference with folates. In the second and third trimesters, use should be avoided, unless clinically necessary. Breastfeeding: Trimethoprim is excreted in breast milk. Effects on the suckling child are likely if therapeutic doses are administered to breast-feeding mothers. Trimethoprim is contraindicated if the breast fed infant is less than 4 months of age.

Trimethoprim (UTI)

Brand names: Monotrim

Trimethoprim is an oral antibacterial widely used as first-line short-course treatment for uncomplicated lower urinary tract infection, where local resistance allows.

Adult dose

Dose: 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.
Route: Oral (oral suspension 50 mg/5 ml)
Frequency: Treatment: twice daily. Long-term prevention of recurrent urinary tract infections: once, at night.
Urinary tract infection is the first indication in the pinned product's §4.1 ("the prevention and treatment of urinary tract infections in adults and children") and §4.2 prints both a treatment and a long-term prevention regimen, so a dose is written. SCOPE: pinned product is the 50 mg/5 ml oral SUSPENSION (doses printed in mg with the equivalent volume). NOT PRINTED in the retrieved §4.2, so not written: any treatment duration, any maximum dose, and any distinction between uncomplicated and complicated or upper and lower urinary tract infection. Children over 12 years take the adult dose; children 6 weeks to 12 years are in paedDose. Renal impairment: reduced-dose schedule by eGFR (see renalAdjustment). Pregnancy: contraindicated in the first trimester; in the second and third trimesters use should be avoided unless clinically necessary (§4.3, §4.6). Trimethoprim is contra-indicated in premature infants and neonates under 6 weeks (§4.3).

Paediatric dose

Dose: Children 6 weeks to 12 years — treatment of urinary tract infections: "based on a dosage of 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 years–12 years 100 mg (10 ml) twice daily. Long-term prevention of recurrent urinary tract infection is based on 2 mg/kg body weight daily given as a single dose at night; suggested regimens: 6 months–5 years 25 mg (2.5 ml) at night; 6 years–12 years 50 mg (5 ml) at night.
Route: Oral (oral suspension 50 mg/5 ml)
Frequency: Treatment: twice daily. Long-term prevention: once daily, at night.
These are the label's own printed figures for urinary tract infection (treatment and long-term prevention) in children 6 weeks to 12 years, not scaled from the adult dose. The age bands and doses come from a flattened table in the retrieved text (the age headings are printed first, then the doses); they are read in the order printed. The retrieved §4.2 prints no paediatric figure for 'other susceptible infections', and no paediatric maximum. Not for premature infants or neonates under 6 weeks (§4.3). Children over 12 years take the adult dose. Product: 50 mg/5 ml oral suspension. "Particular care should be exercised in the haematological monitoring of children on long term therapy." Use a children's formulary for any figure not printed here.

Dose adjustments

Renal

SPC §4.2, 'Dosage advised where there is reduced kidney function' by eGFR (ml/min): over 30 — normal dose; 15–30 — "Normal for 3 days then half dose"; under 15 — "Half the normal dose". "Monitoring of renal function and serum electrolytes should be considered particularly with longer term use, in patients with impaired renal function." "Trimethoprim should only be initiated and used in dialysis patients under close supervision from specialists in both infectious disease and renal medicine. Trimethoprim is removed by dialysis." "Monitoring trimethoprim plasma concentration may be considered with long term therapy but the value of this in individual cases should first be discussed with specialists in infectious disease and renal medicine." Elderly: "Depending on kidney function, see special dosage schedule." §4.4: "In patients with renal impairment, care should be taken to avoid accumulation." No numeric half-dose is printed (the label says only 'half dose' / 'half the normal dose'), so none is calculated here.

Hepatic

Trimethoprim is contra-indicated in severe hepatic insufficiency (§4.3). The retrieved §4.2 prints no dose adjustment for lesser degrees of hepatic impairment. §4.8 reports that "Fatalities have been reported (especially in the elderly, or those with impairment of renal or hepatic function" for the blood and lymphatic reactions, so careful monitoring is advised in those patients.

Dose independently re-sourced from UK Summary of Product Characteristics (SPC) via the eMC — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Hypersensitivity to the active substance or to any of the excipients listed in section 6.1 (§4.3).
  • Severe hepatic insufficiency (§4.3).
  • Megaloblastic anaemia and other blood dyscrasias (§4.3).
  • Pregnant women (§4.3: first trimester of pregnancy; §4.6: contra-indicated in pregnant women, and in the second and third trimesters use should be avoided unless clinically necessary), premature infants and neonates under 6 weeks (§4.3).
  • Breast-fed infant less than 4 months of age (§4.6: "Trimethoprim is contraindicated if the breast fed infant is less than 4 months of age").
  • Known or suspected risk of acute porphyria: treatment should be avoided (§4.4 warning, not listed in §4.3).
  • Previous serious reaction (SJS, TEN or DRESS) to trimethoprim: "the treatment must not be restarted in this patient at any time" (§4.4).
  • Hereditary fructose intolerance: "Patients with rare hereditary problems of fructose intolerance should not take this medicine" (§4.4; this product is the oral suspension).

Side effects

  • Very common: hyperkalaemia. Elderly patients and those taking high doses "may be more susceptible to hyperkalaemia and hyponatraemia" (§4.8).
  • Common: monilial overgrowth; headache; nausea, diarrhoea, vomiting; skin rashes, urticaria (§4.8).
  • Blood and lymphatic (very rare): leucopenia, neutropenia, thrombocytopenia, pancytopaenia, bone marrow depression, agranulocytosis, aplastic anaemia, haemolytic anaemia, eosinophilia, purpura, haemolysis; (not known): megaloblastic anaemia, methaemoglobinaemia, depression of haematopoiesis. Fatalities have been reported, especially in the elderly or those with renal or hepatic impairment; most haematological changes are mild and reversible when treatment is stopped (§4.8).
  • Immune (very rare): hypersensitivity, anaphylaxis, angioedema, drug fever, allergic vasculitis resembling Henoch-Schoenlein purpura, periarteritis nodosa, systemic lupus erythematosus (§4.8).
  • Metabolism (very rare): hypoglycaemia, hyponatraemia, anorexia (§4.8).
  • Psychiatric (very rare): depression, hallucinations, confusional states, agitation, anxiety, abnormal behaviour, insomnia and nightmares (§4.8).
  • Nervous system and sensory (very rare): dyskinesias, aseptic meningitis (rapidly reversible on withdrawal but recurred on re-exposure to co-trimoxazole or trimethoprim alone), tremor, ataxia, dizziness, lethargy, syncope, paraesthesiae, convulsions, peripheral neuritis, vertigo, tinnitus; uveitis (§4.8).
  • Respiratory (very rare): cough, shortness of breath, wheeze, epistaxis (§4.8).
  • Gastrointestinal (very rare): constipation, glossitis, stomatitis, pseudomembranous colitis, pancreatitis; (not known): sore mouth, gastro-intestinal disturbance (§4.8).
  • Hepatobiliary (very rare): elevation of serum transaminases, elevation of bilirubin levels, cholestatic jaundice, hepatic necrosis; cholestatic jaundice and hepatic necrosis may be fatal (§4.8).
  • Skin (very rare): photosensitivity, exfoliative dermatitis, fixed drug eruption, erythema multiforme, erythema nodosum, Stevens-Johnson syndrome, toxic epidermal necrolysis, bullous dermatitis, purpura; (not known): pruritus, drug reaction with eosinophilia and systemic symptoms (DRESS). Toxic epidermal necrolysis "carries a high mortality" (§4.8).
  • §4.8 is truncated at the source-fetch limit after the skin disorders; the musculoskeletal and any later system-organ classes were not retrieved.

Interactions

  • Folate antagonists and anticonvulsants: trimethoprim may induce folate deficiency in predisposed patients, such as those receiving concomitant folate antagonists or anticonvulsants (§4.5).
  • Bone marrow depressants: may increase the potential for bone marrow aplasia; cytotoxics such as azathioprine, mercaptopurine and methotrexate increase the risk of haematologic toxicity; special care with pyrimethamine (§4.5).
  • Phenytoin and digoxin: trimethoprim may increase their plasma concentrations by increasing their elimination half-life; careful monitoring advised (§4.5).
  • Rifampicin may decrease trimethoprim concentrations (§4.5).
  • Diuretics (primarily thiazides) in elderly patients: increased incidence of thrombocytopenia with purpura (§4.5).
  • Drugs that increase serum potassium (potassium-sparing diuretics, potassium supplements, potassium-containing salt substitutes, renin-angiotensin system inhibitors such as ACE inhibitors or angiotensin receptor blockers, heparin): may lead to a significant increase in serum potassium; monitor potassium (§4.4, §4.5).
  • Ciclosporin: increased risk of nephrotoxicity (§4.5).
  • Procainamide: trimethoprim increases plasma concentrations of procainamide (§4.5).
  • Dapsone: plasma concentrations of trimethoprim and dapsone may increase when taken together (§4.5).
  • Repaglinide: trimethoprim may enhance the hypoglycaemic effects of repaglinide; blood glucose monitoring advised (§4.4, §4.5).
  • Warfarin and other coumarins: trimethoprim may potentiate the anticoagulant effect (§4.5).
  • Pyrimethamine: increased antifolate effect (§4.5).

Monitoring

  • Regular haematological tests during long-term therapy and in patients predisposed to folate deficiency (e.g. the elderly), to check for possible pancytopaenia; particular care with haematological monitoring in children on long-term therapy (§4.4). If there is evidence of folic acid deficiency, calcium folinate should be given and response checked by haematologic monitoring (§4.4).
  • Renal function and serum electrolytes: monitoring "should be considered particularly with longer term use, in patients with impaired renal function" (§4.2, §4.4).
  • Serum potassium: close monitoring in patients at risk of hyperkalaemia (renal insufficiency, poorly controlled diabetes mellitus, potassium-sparing diuretics, potassium supplements, potassium-containing salt substitutes, renin angiotensin system inhibitors, heparin) (§4.4).
  • Blood glucose if co-administered with repaglinide (§4.4).
  • Skin: monitor closely for severe cutaneous reactions (SJS, TEN, DRESS); withdraw trimethoprim immediately if signs appear (§4.4).
  • Trimethoprim plasma concentration may be considered with long-term therapy, after discussion with specialists in infectious disease and renal medicine (§4.2).

Clinical monograph

How it works

It inhibits bacterial dihydrofolate reductase, blocking folate synthesis and thereby bacterial DNA and protein production.

Prescribing in practice

  • Avoid in the first trimester of pregnancy because its folate-antagonist action carries a risk of neural tube defects; choose an alternative and give folate where exposure occurs.
  • It can cause hyperkalaemia and raise serum creatinine, with particular caution alongside ACE inhibitors, angiotensin receptor blockers, potassium-sparing agents and in renal impairment.
  • Concurrent use with methotrexate increases the risk of folate-related bone marrow toxicity.

Monitoring

For short courses little monitoring is needed, but check renal function and potassium with prolonged use, in the elderly or with interacting drugs.

Counselling the patient

  • Complete the prescribed course and seek review if symptoms persist or worsen.
  • Tell your prescriber if you are or might be pregnant.

Evidence & guidelines

NICE urinary tract infection guidance lists trimethoprim as an empirical option for lower UTI, with choice guided by local resistance patterns.

Reference: NICE NG112; PHE antimicrobial guidelines; 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.

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