Sulfamethoxazole
in BACTRIM
“Sulfa mask”
Needs a back trim → paired with TMP in Bactrim
What it isSulfonamide blocking dihydropteroate synthase, paired with trimethoprim; urinary infection, MRSA skin infection and PJP.
Why this oneThe sequential blockade with trimethoprim is synergistic, and it is the only oral first-line agent for Pneumocystis.
What limits itHyperkalemia, severe cutaneous reactions, and it raises warfarin effect through CYP2C9. Avoid in late pregnancy and G6PD deficiency.
Sulfamethoxazole prescription history
Animated chart of estimated U.S. prescriptions per year, Antibacterials (systemic) (1938–2026). Sulfamethoxazole enters in 1961 (estimated).
FDA-approved for
- bacterial infections (~1961)
NO BOXED WARNINGS
Principal riskHyperkalemia and severe cutaneous reactions
Sulfonamide
Sulfonamide antimicrobial
The reference sulfonamide antimicrobial: N4 aromatic amine (nitroso-metabolite route to SJS/TEN and DRESS), sulfanilamide SO₂NH, and an N1 isoxazole that gives IgE its specificity. After a reaction, avoid all sulfonamide antimicrobials and trimethoprim; the diuretics, celecoxib, sulfonylureas and triptans are unlikely to cross-react.
Open the sulfonamide hypersensitivity presentation → · sources [n] are on its References slide.
Classes and tags
Clinical profile
Defining liabilitySulfonamide hypersensitivity and crystalluria
Open Sulfamethoxazole in the interactive console ↗
Memory aids: interaction avatars
Open Sulfamethoxazole in the interactive console ↗
Self-check
- Which enzyme does sulfamethoxazole block?
Dihydropteroate synthase. - Through which enzyme does sulfamethoxazole raise warfarin effect?
CYP2C9.