Medications and Drugs

GENERIC NAME: sulfamethoxazole

BRAND NAME: Gantanol

DRUG CLASS AND MECHANISM: 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.

PRESCRIPTION: yes

GENERIC AVAILABLE: yes

PREPARATIONS: Tablets (light green; round): 500 mg.

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

PRESCRIBED FOR: Sulfamethoxazole is used for the treatment of malaria (in combination with quinine sulfate and pyrimethamine), conjunctivitis (inflammation of the conjunctiva of the eye) due to chlamydia, toxoplasmosis (in combination with pyrimethamine), and urinary tract infections (UTI).

DOSING: 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.

DRUG INTERACTIONS: 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 causes an acidic urine, it should not be used with sulfonamides.

PREGNANCY: The effects of the sulfonamide class of antibiotics on the fetus have not been adequately studied. Physicians may elect to 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.

NURSING MOTHERS: Sulfamethoxazole is excreted in breast milk and should not be used in women who are breast-feeding. Sulfonamides can cause kernicterus in nursing newborns.

SIDE EFFECTS: Sulfamethoxazole may cause 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 rare side effects include liver damage, low white blood cell count, low platelet count, 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.

Reference: FDA Prescribing Information

Last Editorial Review: 12/31/1997

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