What Is Trimethoprim and Sulfamethoxazole?

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  • Written By: Jacquelyn Gilchrist
  • Edited By: O. Wallace
  • Last Modified Date: 10 October 2019
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Trimethoprim and sulfamethoxazole are antibiotics that are often combined into a single medication. A doctor may prescribe this drug to treat bacterial infections, such as intestinal, respiratory, or urinary tract infections. This medicine can also treat middle ear infections, traveler's diarrhea, and a type of pneumonia. It cannot be used to treat viral infections. Trimethoprim and sulfamethoxazole work by interfering with specific proteins that many types of bacteria need to flourish.

The typical dose for adults will vary, depending on what specific condition is being treated and in some cases, the weight of the patient. For example, those taking it for pneumocystis pneumonia will usually be prescribed 160 to 180 milligrams (mg) once daily. Patients with bronchitis may take between 160 to 800 mg once every 12 hours, often for a total of five days. Trimethoprim and sulfamethoxazole should be taken with a full glass of water and patients should continue to consume plenty of fluids to help prevent kidney stones, which are a possible complication of the drug. It may also be taken with food, especially if the patient experiences an upset stomach.


Some side effects may occur while using trimethoprim and sulfamethoxazole, which should be reported to the prescribing physician if they become severe. Patients may experience insomnia, a painful or swollen tongue, or joint pain. Nausea, vomiting, and diarrhea have also been reported. Other side effects can include a headache, loss of appetite, and ringing in the ears. Vertigo or the sensation of spinning have occasionally been reported.

Certain side effects may require medical care. Patients should see their doctors if they experience symptoms of yeast infections, which may be oral or vaginal yeast infections. These can include unusual vaginal discharge, itching, and white patches in the mouth. Taking the drug with yogurt that contains active bacteria cultures may help prevent this side effect.

Rarely, serious side effects may occur that require emergency medical help. These can include problems breathing, persistent nausea or vomiting, and dark urine. Trimethoprim and sulfamethoxazole can also cause blood or mucus in the stool, abdominal cramping, and persistent diarrhea. Anti-diarrhea products should not be used, because they can worsen this condition. Rarely, this drug may result in complications like blood disorders, lung or liver damage, and Stevens-Johnson syndrome.

Before using these substances to treat a bacterial infection, patients must disclose their other medical conditions, medications, and supplements. Women who are pregnant or breastfeeding should avoid this drug. Trimethoprim and sulfamethoxazole may be contraindicated for use by those who have asthma, kidney or liver disease, or those who frequently use alcohol. It may interact with other medicines, including cyclosporine, blood thinners, and diabetes drugs.


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