Sulfamethoxazole

Sulfamethoxazole (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 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,  and sulfamethazole.

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. 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 folic acid enzyme that inhibit PABA from binding and block folic acid synthesis.

A TMP-SMZ (also TMP-SMX or TMP-Sulfa) disk is a combination of trimethoprim and sulfamethoxazole 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, severe stomach, or abdomen pains. Headaches commonly occur when taking sulfamethoxazole. Muscle pains sometimes occur when taking this medicine. 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 medicine and get emergency medical help. These are often symptoms of a severe allergic reaction.

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