DRUG OF THE DAY
trimethoprim/sulfamethoxazole
Folate-antagonist antibacterial combination
Before opening the card, recall one use, one safety check and one counseling point.
Reveal the drug learning card
Names and formulations
- Bactrim / Bactrim DS: single-strength and double-strength tablets contain different amounts of both components
Dose is often expressed by the trimethoprim component; one DS tablet contains twice the active ingredients of one single-strength tablet.
Common use
Treatment of selected susceptible infections such as urinary tract infection and treatment or prevention of Pneumocystis jirovecii pneumonia; indication and dose differ substantially.
Mechanism of action
Sequentially inhibits bacterial folate synthesis: sulfamethoxazole blocks dihydropteroate synthase and trimethoprim blocks dihydrofolate reductase.
Important adverse effects and monitoring
- Can cause severe cutaneous reactions, blood dyscrasias, hyperkalemia, kidney injury, and hypersensitivity; stop and evaluate promptly for rash or systemic toxicity.
- Monitor kidney function, potassium, and CBC when patient factors, dose, or duration increase risk.
- Contraindications and pregnancy/newborn restrictions are specific; verify the current label before use.
Patient counseling
- Maintain adequate fluid intake unless fluid restriction has been prescribed.
- Report rash, mouth sores, fever, unusual bruising, severe weakness, or reduced urine output promptly.
Interaction to recognize
With: warfarin
Why it matters: Can increase INR and bleeding risk.
Practical response: Arrange closer INR monitoring and warfarin management when the combination cannot be avoided.
Check the original sources
- DailyMed — Bactrim and Bactrim DS prescribing information
Sections: Clinical Pharmacology; Indications and Usage; Contraindications; Warnings; General; Information for Patients; Drug Interactions; Dosage and Administration. Source checked 2026-09-13.
Selected learning points; verify current labeling for the complete prescribing information.