Sulfamethoxazole
A sulfonamide antibiotic, given almost always with trimethoprim as co-trimoxazole, for urinary infections and Pneumocystis pneumonia.
Also called co-trimoxazole (name of its combination with trimethoprim), TMP-SMX (abbreviation for the combination) or SMX (abbreviation).
Drug card
- How it works
- Blocking dihydropteroate synthase, so bacteria cannot make folate
- Used for
- Pneumocystis pneumonia: treatment and prevention
- Urinary tract infections
- Shigella enteritis and traveler's diarrhea
- Middle ear infections in children
- Known for
- Stevens-Johnson syndrome and other severe rashes
- Its name
- sulfa- means a sulfonamide anti-infective
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does sulfamethoxazole work?
A sulfonamide that competes with para-aminobenzoic acid (PABA) for dihydropteroate synthase, so bacteria cannot make dihydrofolic acid. On its own it only stops growth (bacteriostatic), so it is used almost always with trimethoprim, which blocks the next step, as co-trimoxazole.
Like other antibacterial folate antagonists: Bacteria must build their own folate to make the nucleic acids and proteins they grow on. These drugs block that pathway at two consecutive steps: sulfamethoxazole stops dihydrofolic acid being made, and trimethoprim stops it being turned into tetrahydrofolic acid. Given together as co-trimoxazole they act in synergy, and resistance develops more slowly than to either alone.
Uses
What is sulfamethoxazole used for?
- Pneumocystis pneumonia: treatment and prevention
- Urinary tract infections
- Shigella enteritis and traveler's diarrhea
- Middle ear infections in children
Side effects
Side effects of sulfamethoxazole
- Stevens-Johnson syndrome and other severe rashesKnown forSerious
- Rash and hives
- Nausea and vomiting
- AgranulocytosisSerious
- Aplastic anemiaSerious
- Hemolysis in G6PD deficiencySerious
- Liver injury, rarely liver necrosisSerious
- Crystals in the urine and kidney injurySerious
- Low blood sugarSerious
- Sensitivity to sunlight
- Acute lung injury and breathing failureSerious
- C. difficile diarrheaSerious
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- Contraindicated under 2 months of age: sulfonamides can displace bilirubin and risk kernicterus in newborns.
- Strengthens warfarin and raises phenytoin levels by blocking CYP2C9, so INR and phenytoin are checked.
- Stop at the first rash: it can lead on to Stevens-Johnson syndrome or TEN, which can kill.
- Keep fluid intake up during treatment to prevent crystals in the urine.
Good to know: Almost always given with trimethoprim as co-trimoxazole (TMP-SMX), the combination studied for every listed use; the two drugs block consecutive steps of folate synthesis.
Remember it
A way to remember it
Double lock on folate: SMX shuts step one, TMP shuts step two.
Sulfamethoxazole (SMX) blocks dihydropteroate synthase and trimethoprim (TMP) blocks dihydrofolate reductase, the next step in making folate. Two blocks in a row work in synergy and slow resistance, which is why they are given together as co-trimoxazole.
Written for biofideX
Its name
sulfa- means sulfonamide anti-infective
Sulfamethoxazole and sulfadiazine share it. Sulfasalazine, the anti-inflammatory parent drug of mesalazine, carries it too.
- Every drug name stem
Its class
Antibacterial folate antagonists
Other antibacterial folate antagonists with a card:
Easily mixed up with antimetabolites and nitrofurans.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Sulfamethoxazole and trimethoprim tablets: label (DailyMed)
- Sulfamethoxazole (NCI Thesaurus)
- Trimethoprim-Sulfamethoxazole (NCI Thesaurus)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Antibiotics: DNA, folate and tuberculosis
Sulfamethoxazole is one of 11 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
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