Sulfonamides are medicines that prevent the growth of bacteria in the body. They are used to treat many kinds of infections caused by bacteria and certainother microorganisms. Doctors may prescribe these drugs to treat urinary tract infections, ear infections, frequent or long-lasting bronchitis, bacterialmeningitis, certain eye infections, Pneumocystis carinii pneumonia, traveler's diarrhea, and a number of other kinds of infections. These drugs will not work for colds, flu, and other infections caused by viruses.

Sulfonamides (also called sulfa drugs) are available only by prescription intablet and liquid forms. Some commonly-used sulfonamides are sulfisoxazole (Gantrisin) and the combination drug sulfamethoxazole and trimethoprim (Bactrim, Cotrim).

The recommended dosage depends on the type of sulfonamide, the strength of the medicine, and the medical problem for which it's being taken. Sulfonamidesshould always be taken exactly as directed and for the full course of medication to make sure the infection clears up completely.

Sulfonamides work best when they are at constant levels in the blood, so themedicine should be taken in evenly-spaced doses through the day and night. For best results, the medicine should be taken with a full glass of water. Drinking extra glasses of water throughout the day may help prevent side effects.

Although such side effects are rare, some people have had severe and life-threatening reactions to sulfonamides. These include sudden, severe liver damage, serious blood problems, breakdown of the outer layer of the skin, and a condition called Stevens-Johnson syndrome, in which people get blisters around the mouth, eyes, or anus. Call a doctor right away if any of these signs of adangerous reaction occur:

  • Skin rash or reddish or purplish spots on the skin
  • Other skin problems, such as blistering or peeling
  • Fever
  • Sore throat
  • Cough
  • Shortness of breath
  • Joint pain
  • Pale skin
  • Yellow skin or eyes

Because this medicine may cause dizziness, anyone who takes sulfonamides should not drive, use machines or do anything that might be dangerous until theyhave found out how the drugs affect them.

Sulfonamides may cause blood problems that can interfere with healing and lead to additional infections, so injuries should be avoided while taking this medicine. It's especially important not to damage the mouth when brushing or flossing the teeth or using a toothpick. Dental work should not be done untilthe blood is back to normal.

This medicine may increase sensitivity to sunlight. Since even brief exposureto sun can cause a severe sunburn or a rash, patients should avoid direct sunlight, especially between 10 a.m. and 3 p.m.; wear a hat and tightly woven clothing that covers the arms and legs; use a sunscreen with a skin protectionfactor (SPF) of at least 15; protect the lips with a sun block lipstick; andavoid tanning beds, tanning booths, or sunlamps.

Babies under 2 months should not be given sulfonamides unless their doctor has ordered the medicine. Older people may be especially sensitive to the effects of sulfonamides, increasing the chance of unwanted side effects, such as severe skin problems and blood problems. Patients who are taking water pills (diuretics) at the same time as sulfonamides may also be more likely to have these problems.

People with certain medical conditions or who are taking certain other medicines can have problems if they take sulfonamides:


Anyone who has had unusual reactions to sulfonamides, water pills (diuretics), diabetes medicines, or glaucoma medicine in the past should let his or herphysician know before taking sulfonamides. The physician should also be toldabout any allergies to foods, dyes, preservatives, or other substances.


In studies of laboratory animals, some sulfonamides cause birth defects. Thedrugs' effects on unborn babies have not been studied, but pregnant women should not use this medicine near the time of labor and delivery because it maycause side effects in the baby. Women who are pregnant or who may become pregnant should check with their doctors about the safety of using sulfonamides during pregnancy.


Sulfonamides pass into breast milk and may cause liver problems, anemia, andother problems in nursing babies whose mothers take the medicine. Because ofthose problems, women should not breastfeed when they are taking this drug. Women who are breastfeeding and who need to take this medicine should check with their doctors to find out how long they need to stop breastfeeding.

Other medical conditions

Before using sulfonamides, people with any of these medical problems should make sure their physicians are aware of their conditions:

  • Anemia or other blood problems
  • Kidney disease
  • Liver disease
  • Asthma or severe allergies
  • Alcohol abuse
  • Poor nutrition
  • Abnormal intestinal absorption
  • Porphyria (a disorder of blood pigment)
  • Folic acid deficiency
  • Deficiency of the enzymeglucose-6-phosphate dehydrogenase (G6PD).

Taking sulfonamides with certain other drugs may affect the way the drugs work or may increase the chance of side effects. The most common side effects are mild diarrhea, nausea, vomiting, dizziness, headache, loss of appetite, and tiredness. These problems usually go away as the body adjusts to thedrug and don't require medical treatment. More serious side effects aren't common, but any of the following should be reported to a doctor immediately:

  • Itching or skin rash
  • Reddish or purplish spots on the skin
  • Other skin problems, such as redness, blistering, peeling
  • Severe, watery or bloody diarrhea
  • Muscle or joint aches
  • Fever
  • Sore throat
  • Cough
  • Shortness of breath
  • Unusual tiredness or weakness
  • Unusual bleeding or bruising
  • Paleskin
  • Yellow eyes or skin
  • Swallowing problems.

Sulfonamides may interact with a large number of other medicines. When this happens, the effects of one or both of the drugs may change or the risk of side effects may be greater. Among the drugs that may interact with sulfonamidesare:

  • Acetaminophen (Tylenol)
  • Medicine for overactive thyroid
  • Male hormones (androgens)
  • Female hormones (estrogens)
  • Other medicines used to treat infections
  • Birth control pills
  • Medicines for diabetes such as glyburide (Micronase)
  • Blood thinners such as warfarin (Coumadin)
  • Disulfiram (Antabuse), used to treat alcohol abuse
  • Amantadine (Symmetrel) used to treat flu and alsoParkinson's disease
  • Water pills (diuretics) such as hydrochlorothiazide (HCTZ, HydroDIURIL)
  • The anticancer drug methotrexate (Rheumatrex)
  • Antiseizure medicines such as valproic acid (Depakote, Depakene).

The list above does not include every drug that may interact with sulfonamides, so any patient taking this medicine should check with a doctor or pharmacist before combining sulfonamides with anything.

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