Trimethoprim

Dihydrofolate Reductase Inhibitor·Infectious Diseases - Urinary Agents

Acute uncomplicated lower UTI, prophylaxis of recurrent UTI

PBSANTIBIOTICAlprim, Triprim300 mg tabletS4+1

Max Dose

300 mg/day

Half-life

8-10 h

Hyperkalemia

RISK

Creatinine

ARTIFICIAL BUMP

Rapid Summary

Overview

Folate antagonist. Used as monotherapy exclusively for uncomplicated UTIs. Avoids the severe skin/sulfa toxicities of Bactrim (Co-trimoxazole).

Maximum Dose

Max 300 mg/day.

Clinical Focus

Blocks potassium excretion. Extremely dangerous if given to elderly patients on ACEi/ARBs or Spironolactone.

Bottom Line

Trimethoprim is a simple first-line antibiotic for uncomplicated UTIs, but causes hyperkalaemia (especially with ACEi/ARBs), falsely elevates creatinine, and folate antagonism limits use in pregnancy.

Formulation & Access

Brand Names

Alprim, Triprim

Oral Routes

Tablets (300 mg).

Prescribing & PBS

Unrestricted General Benefit (S4).

Indications & Use

Primary

PBSTGA

Acute uncomplicated lower UTI, prophylaxis of recurrent UTI.

Clinical Place

First-line oral agent for cystitis alongside Nitrofurantoin and Cephalexin.

Quick reference

Class & Role

Dihydrofolate Reductase Inhibitor — Folate antagonist.

Route / Formulation

Tablets (300 mg). (Alprim, Triprim)

Usual Dose & Max

PO 300 mg OD at night for 3-5 days. Max 300 mg/day.

Key Indication Doses

Acute Uncomplicated UTI: PO 300 mg OD at night for 3-5 days. UTI Prophylaxis: PO 150 mg NOCTE. Renal Impairment: Halve the dose if eGFR < 30. Avoid if eGFR < 15.

Best Uses

Avoid / Cautions

Severe renal impairment without dose adjustment, megaloblastic anemia from folate deficiency. Pregnancy Category B3. Strictly avoid in 1st trimester as it blocks folate, massively increasing the risk of neural tube defects (spina bifida).

Key Risks

Renal & Electrolytes: Hyperkalemia (acts like a potassium-sparing diuretic in the distal tubule). Artificial elevation of serum creatinine. Gastrointestinal: Nausea, vomiting, glossitis. Haematological: Bone marrow depression, megaloblastic anemia (due to folate blockade, mostly with prolonged use).

Key Interactions

ACEi, ARBs, Spironolactone. The combination guarantees hyperkalemia in the elderly. Check K+ levels. Methotrexate. Co-administration causes fatal bone marrow failure.

Monitoring

U&E (potassium) if the patient is elderly or on RAAS blockers. Ensure the patient is not taking over-the-counter K+ supplements.

Clinical Pearl

Trimethoprim competitively blocks the exact same tubular transporter that secretes creatinine. Blood creatinine will artificially rise by ~20% within days. This is a harmless lab artefact. If the Urea is normal and K+ is stable, do not panic and diagnose AKI.

Access

Alprim, Triprim300 mg tablet

Brand: Alprim, Triprim
Formulation: Tablets (300 mg)