What is sulfamethoxazole, and how does it work (mechanism of action)?

Sulfamethoxazole is an anti- bacterial sulfonamide. It prevents the formation of dihydrofolic acid, a compound that bacteria must be able to make in order to survive. Although it was once a very useful antibiotic, it is almost obsolete as a single agent today due to the development of bacterial resistance to its effects. Sulfamethoxazole is now used primarily in combination with trimethoprim, a combination product known as Bactrim or Septra. Sulfamethoxazole was approved by the FDA in 1961. According to the FDA database, all brand and generic formulations of sulfamethoxazole have been discontinued.

Is sulfamethoxazole available as a generic drug?

GENERIC AVAILABLE: Yes

Do I need a prescription for sulfamethoxazole?

Yes

What are the side effects of sulfamethoxazole?

: Common side effects of sulfamethoxazole are:

• dizziness,
• headache,
• lethargy,
• diarrhea,
• anorexia,
• nausea,
• vomiting, and
• rash.

Sulfamethoxazole should be stopped at the first appearance of a skin rash since the rash may become severe. Serious rashes include Stevens-Johnson syndrome (aching joints and muscles; redness, blistering, and peeling of the skin); toxic epidermal necrolysis (difficulty in swallowing; peeling, redness, loosening, and blistering of the skin). Sulfamethoxazole therapy also can cause extensive sunburn, following exposure to sunlight. Patients receiving sulfamethoxazole should avoid excessive exposure to sunlight and should wear sunscreen.

Other important rare side effects include:

• liver damage,
• low white blood cell count,
• low platelet count (thrombocytopenia), and
• anemia.

Sulfamethoxazole may form crystals in the urine which may damage the kidney and cause bleeding into the urine. It is important to drink additional liquids during sulfonamide therapy to prevent these side effects.

What is the dosage for sulfamethoxazole?

Sulfamethoxazole usually is taken two or three times daily, with or without meals. It should be taken with 6 to 8 ounces of liquid to prevent crystals from forming in the urine. Persons with advanced kidney diseases may require lower doses.

Which drugs or supplements interact with sulfamethoxazole?

Sulfamethoxazole can enhance the blood-thinning effects of warfarin (Coumadin), possibly leading to bleeding. Sulfonamides such as sulfamethoxazole can increase the metabolism (break-down and elimination) of cyclosporine (causing loss of effectiveness of cyclosporine), and can add to the kidney damage caused by cyclosporine. All sulfonamides can crystallize in urine when the urine is acidic. Since methenamine (Hiprex, Urex, Mandelamine) causes an acidic urine, it should not be used with sulfonamides.

Is sulfamethoxazole safe to take if I'm pregnant or breastfeeding?

The effects of the sulfonamide class of antibiotics on the fetus have not been adequately studied. Therefore, physicians may use them if the benefits are deemed to outweigh potential risks. On the other hand, use of sulfonamides near term (that is, near the ninth month of pregnancy) may cause bilirubin to be displaced from proteins in the infant's blood. Displacement of bilirubin can lead to a dangerous condition called kernicterus in which the bilirubin damages the brain. For this reason, sulfonamides should not be used near term birth.

Sulfamethoxazole is excreted in breast milk and should not be used in women who are breastfeeding. Sulfonamides can cause kernicterus in nursing newborns.

What else should I know about sulfamethoxazole?

What preparations of sulfamethoxazole are available?

Tablets: 500 mg and 1 gm.

How should I keep sulfamethoxazole stored?

The tablets should be kept at room temperature, 15 C - 30 C (59 F - 86 F).