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Systematic (IUPAC) name
CAS number 723-46-6
ATC code J01EC01
PubChem 5329
DrugBank APRD00076
Chemical data
Formula C10H11N3O3S 
Mol. mass 253.279 g/mol
SMILES search in eMolecules, PubChem
Physical data
Melt. point 3 °C (37 °F)
Pharmacokinetic data
Bioavailability  ?
Protein binding 70%
Metabolism Hepatic acetylation and glucuronidation
Half life 10 hours
Excretion Renal
Therapeutic considerations
Pregnancy cat.

C (Au, U.S.)

Legal status

Prescription Only (S4)(AU) ℞-only

Routes Oral

Sulfamethoxazole is a sulfonamide bacteriostatic antibiotic. It is most often used as part of a synergistic combination with trimethoprim in a 5:1 ratio in co-trimoxazole, which is also known as Bactrim, Septrin, or Septra (also abbreviated SMX/TMP). Its primary activity is against susceptible forms of Streptococcus, Staphylococcus aureus, Escherichia coli, Haemophilus influenzae, and oral anaerobes. It is commonly used to treat urinary tract infections. In addition can be used as an alternative to amoxicillin-based antibiotics to treat sinusitis.


Mechanism of action

Sulfonamides are structural analogs and competitive antagonists of para-aminobenzoic acid (PABA). They inhibit normal bacterial utilization of PABA for the synthesis of folic acid, an important metabolite in DNA synthesis.[1] The effects seen are usually bacteriostatic in nature. Folic acid is not synthesized in humans, but is instead a dietary requirement. This allows for the selective toxicity to bacterial cells (or any cell dependent on synthesizing folic acid) over human cells. Bacterial resistance to sulfamethoxazole are caused by mutations in the folic acid enzyme that prevents the drug from binding and blocking folic acid synthesis.


Side effects

The most common side effect of sulfamethoxazole/trimethoprim is gastrointestinal upset. Allergies to sulfa-based medications typically cause skin rashes, hives, or trouble breathing or swallowing and warrant immediate discontinuation of the medication and contact with doctor immediately. Sulfamethoxazole/trimethoprim is also known to increase blood concentrations of the drug warfarin (U.S. brand name: Coumadin) and can cause an unexpected increase in clotting time and uncontrolled bleeding. Neutropenia and thrombocytopenia also are rare adverse effects to be monitored if a patient is placed on long-term therapy.


  1. ^ Martindale, The extra pharmacopoeia, 30th ed, p. 208

See also

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Sulfamethoxazole". A list of authors is available in Wikipedia.
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