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Sulfamethoxazole (also spelled sulphamethoxazole (BRIT)) (abbreviated SMZ or SMX)  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, 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 strains of Streptococcus, Staphylococcus aureus (including methicillin resistant strains), 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 and sulfamethazole.

Side effects
The most common side effect of sulfamethoxazole/trimethoprim is gastrointestinal upset (reference needed). 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.

Mechanism of action
Sulfamethoxazole, a sulfonamide, is a structural analog of para-aminobenzoic acid (PABA). They compete with PABA to bind to dihydropteroate synthetase and inhibit conversion of PABA and dihydropteroate diphosphate to dihydrofolic acid, or dihydrofolate. Inhibiting the production of dihydrofolate intermediate interferes with the normal bacterial synthesis of folic acid (folate). Folate is an essential metabolite for bacterial growth and replication because it is used in DNA synthesis, primarily at thymidylate and purine biosynthesis, and amino acids synthesis, including serine, glycine and methionine. Hence, blockage of folate production inhibits the folate-dependent metabolic processes for bacterial growth and makes sulfamethoxazole a bacteriostatic antibiotic. Sulfonamides are selective against bacteria because it interferes with the synthesis of folate, a process which does not occur in humans.