Sulfonamide + Trimethoprim·Infectious Diseases - Other Antibiotics

Co-trimoxazole

MRSA skin infections, Pneumocystis jirovecii (PCP), Toxoplasmosis, complicated UTI

Authority requiredPBSSULFONAMIDEBactrim, Resprim. Available as Single Strength (400/80) or Double Strength (DS) (800/160).Tablets, Oral Liquid+2

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At a glance

Max Dose

Weight Based

Half-life

10 h

High risk: Hyperkalemia

HIGH RISK

High risk: SJS/TEN

CRITICAL

Cautions

Avoid / Cautions

Sulfa allergy, severe hepatic/renal failure, folate deficiency. Pregnancy Category C. Avoid in 1st trimester (blocks folate, causing neural tube defects) and near term (kernicterus).

Where it stands

Rapid Summary

Overview

Synergistic combination of Sulfamethoxazole and Trimethoprim (Bactrim). Devastating to MRSA and Pneumocystis (PCP), but highly toxic to the kidneys and skin.

Clinical Focus

Acts identically to a potassium-sparing diuretic (Amiloride). Will cause lethal hyperkalemia if given to elderly patients on ACEi/ARBs.

Bottom Line

Co-trimoxazole is the first-line treatment and prophylaxis for Pneumocystis jirovecii (PJP) and selected UTIs, but causes severe hyperkalaemia, bone marrow suppression, and life-threatening skin reactions (SJS/TEN).

Formulation & Access

Brand Names

Bactrim, Resprim. Available as Single Strength (400/80) or Double Strength (DS) (800/160).

Oral Routes

Tablets, Oral Liquid.

Parenteral

Ampoules for IV infusion (Requires massive dilution volumes).

Prescribing & PBS

Authority Required (PBS) for specific indications.

Indications & Use

Primary

PBSTGA

MRSA skin infections, Pneumocystis jirovecii (PCP), Toxoplasmosis, complicated UTI.

Clinical Place

First-line oral option for severe community MRSA. Anchor drug in HIV/immunocompromised patients for PCP.

Quick reference

Class & Role

Sulfonamide + Trimethoprim — Synergistic combination of Sulfamethoxazole and Trimethoprim (Bactrim).

Route / Formulation

Tablets, Oral Liquid. (Bactrim, Resprim. Available as Single Strength (400/80) or Double Strength (DS) (800/160).)

Usual Dose & Max

PO 1 DS tablet (800/160 mg) BD. Variable. Standard max is 4 Double Strength (DS) tablets/day, but PCP doses are massive.

Key Indication Doses

UTI / MRSA Skin Infection: PO 1 DS tablet (800/160 mg) BD. PCP Prophylaxis: PO 1 DS tablet 3 times per week. PCP Treatment (Severe): IV 120 mg/kg/day (of combined product) in 4 divided doses (Massive volume, ICU monitoring). Renal Impairment: Halve dose if eGFR 15-30. Avoid entirely if eGFR < 15.

Best Uses

Key Risks

Renal & Electrolytes: Severe hyperkalemia (blocks K+ excretion), AKI, crystalluria. Dermatological: Stevens-Johnson Syndrome (SJS), Toxic Epidermal Necrolysis (TEN). Haematological: Bone marrow suppression (leukopenia, thrombocytopenia, megaloblastic anemia due to folate antagonism).

Key Interactions

ACEi, ARBs, Spironolactone (Guarantees severe hyperkalemia). Methotrexate (Both block folate synthesis. Combo causes fatal bone marrow failure). Warfarin (Drastically increases INR).

Monitoring

Strict U&E monitoring within 3 days of starting in the elderly (potassium spike). Monitor FBC. Cease immediately and admit to ED if ANY mucosal blistering, rash, or unexplained fever occurs.

Clinical Pearl

Trimethoprim competitively blocks the tubular secretion of creatinine in the kidney. Within days of starting, the patient's blood creatinine will rise by 10-30%. If the Urea is stable, this is a harmless lab artefact, NOT a true AKI.

Access

Authority requiredBactrim, Resprim. Available as Single Strength (400/80) or Double Strength (DS) (800/160).Tablets, Oral Liquid

Brand: Bactrim, Resprim. Available as Single Strength (400/80) or Double Strength (DS) (800/160).
PBS status: Authority required
Formulation: Tablets, Oral Liquid.

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