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Clinical data
AHFS/Drugs.comMicromedex Detailed Consumer Information
  • AU: C
  • US: C (Risk not ruled out)
Routes of
Oral, IV
ATC code
Legal status
Legal status
Pharmacokinetic data
Protein binding70%
MetabolismHepatic acetylation and glucuronidation
Elimination half-life10 hours
CAS Number
PubChem CID
E number{{#property:P628}}
ECHA InfoCard{{#property:P2566}}Lua error in Module:EditAtWikidata at line 36: attempt to index field 'wikibase' (a nil value).
Chemical and physical data
Molar mass253.279 g/mol
3D model (JSmol)
Melting point169 °C (336.2 °F)

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Sulfamethoxazole (abbreviated SMZ or SMX)[1][2][3] 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 (abbreviated SMZ-TMP and SMX-TMP,[4] or TMP-SMZ and TMP-SMX), also known under trade names such as Bactrim, Septrin, or Septra; in Eastern Europe it is marketed as Biseptol. Its primary activity is against susceptible forms of Streptococcus, Staphylococcus aureus (including MRSA), Escherichia coli, Haemophilus influenzae, and oral anaerobes. It is commonly used to treat urinary tract infections. In addition it can be used as an alternative to amoxicillin-based antibiotics to treat sinusitis. It can also be used to treat toxoplasmosis and it is the drug of choice for Pneumocystis pneumonia, which affects primarily patients with HIV.

Other names include: sulfamethylisoxazol, sulfisomezole, MS 53, RO 4 2130[5][6] and sulfamethazole.[7]

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.[8] 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 is caused by mutations in the enzymes involved in folic acid synthesis that prevent the drug from binding to it.


A TMP-SMZ (also TMP-SMX or TMP-Sulfa) disk is a combination of trimethoprim and sulfamethoxazole(combination of both is known as cotrimoxazole) that acts synergistically for bactericidal action. A bacterial culture impregnated with a trimethoprim and sulfamethoxazole disk may be used to help identify an organism as Gardnerella vaginalis; it is sensitive to the TMP-SMZ disk.

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. Sulfamethoxazole is also a Stevens-Johnson syndrome (SJS) inducing substance.

Sulfamethoxazole can also cause nausea or severe stomach or abdominal pain. Headaches commonly occur when taking sulfamethoxazole. Muscle pain sometimes occurs when taking this medication. If symptoms persist, one should contact his/her physician. If trouble breathing or swelling of the face, mouth, or tongue occurs, one should discontinue the medication and get emergency medical help. These are often symptoms of a severe allergic reaction (anaphylaxis).

Sulfamethoxazole/trimethoprim can lead to a megaloblastic anemia in some patients because it is a folate antagonist.[9]

See also


  1. Ma, M.; Cheng, Y.; Xu, Z.; Xu, P.; Qu, H.; Fang, Y.; Xu, T.; Wen, L. (2007). "Evaluation of polyamidoamine (PAMAM) dendrimers as drug carriers of anti-bacterial drugs using sulfamethoxazole (SMZ) as a model drug". European journal of medicinal chemistry. 42 (1): 93–8. doi:10.1016/j.ejmech.2006.07.015. PMID 17095123.
  2. Garg, S.K.; Ghosh, S.S.; Mathur, V.S. (1986). "Comparative pharmacokinetic study of four different sulfonamides in combination with trimethoprim in human volunteers". International journal of clinical pharmacology, therapy, and toxicology. 24 (1): 23–5. PMID 3485584.
  3. SMZ in Abstract of "Rat model of concurrent Pneumocystis carinii (Pc), Toxoplasma gondii (Tg), and Mycobacterium avium complex (MAC) infections for assessment of multiple prophylaxis" at
  4. SMZ-TMP in Abstract of "Cutaneous hypersensitivity to sulfamethoxazole-trimethoprim (SMZ TMP) in HIV infected patients" at
  5. PubChem. "Sulfamethoxazole - Substance Summary", PubChem, National Center for Biotechnology Information (NCBI), National Library of Medicine (NLM), National Institutes of Health (NIH)
  6. ChemDB. "Sulfamethoxazole", ChemDB, National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH)
  7. Sulfamethazole in "Clinical Diabetes: Case Study: A 90-Year-Old Man With Confusion and Night Sweats", and "Chronic Granulomatous Disease"
  8. Martindale, The extra pharmacopoeia, 30th ed, p. 208
  9. USMLE World Step1, Qbank Pharmacology, 2009, Q106