Sulfonamides are a group of anti-infective drugs that prevent the growth of bacteria in the body by interfering with their metabolism. Bacteria are one-celled disease-causing microorganisms that commonly multiply by cell division.


Sulfonamides are used to treat many kinds of infections caused by bacteria and certain other microorganisms. Physicians may prescribe these drugs to treat urinary tract infections, ear infections, frequent or long-lasting bronchitis, bacterial meningitis, certain eye infections, Pneumocystis carinii pneumonia (PCP), traveler's diarrhea, and a number of other infections. These drugs will, however, not work for colds, flu, and other infections caused by viruses.


Sulfonamides, which are also called sulfa medicines, are available only with a physician's prescription. They are sold in tablet and liquid forms. Some commonly used sulfonamides are sulfisoxazole (Gantrisin) and the combination drug sulfamethoxazole and trimethoprim (Bactrim, Cotrim, Septra).

Although the sulfonamides have been largely replaced by antibiotics for treatment of infections, some bacteria have developed resistance to antibiotics but can still be treated with sulfonamides because the bacteria have not been exposed to these drugs in the past.

Silver sulfadiazine, an ointment containing a sulfonamide, is valuable for the treatment of infections associated with severe burns. The combination drug trimethoprim/sulfamethoxazole (TMP-SMZ) remains in use for many infections, including those associated with HIV infection (AIDS). TMP-SMZ is particularly useful for prevention and treatment of Pneumocystis carinii pneumonia, which has been the most dangerous of the infections associated with HIV infection.

Recommended dosage

The recommended dosage depends on the type of sulfonamide, the strength of the medication, and the medical problem for which it is being taken. Patients should check the correct dosage with the physician who prescribed the drug or the pharmacist who filled the prescription.

Patients should always take sulfonamides exactly as directed. To make sure the infection clears up completely, the full course of the medicine must be taken. Patients should not stop taking the drug just because their symptoms begin to improve, because the symptoms may return if the drug is stopped too soon.

Sulfonamides work best when they are at constant levels in the blood. To help keep blood levels constant, patients should take the medicine in doses spaced evenly through the day and night without missing any doses. For best results, sulfa medicines should be taken with a full glass of water, and the patient should drink several more glasses of water every day. This precaution is necessary because sulfa drugs do not dissolve in tissue fluids as easily as some other anti-infective medications. Drinking plenty of water will help prevent some of the medicine's side effects.


Symptoms should begin to improve within a few days of beginning to take a sulfa drug. If they do not, or if they get worse, the patient should consult the physician who prescribed the medicine.

Although major side effects are rare, some people have had severe and life-threatening reactions to sulfonamides. These include sudden and severe liver damage; serious blood problems; breakdown of the outer layer of the skin; and a condition called Stevens-Johnson syndrome (erythema multiforme), in which people get blisters around the mouth, eyes, or anus. The patient may be unable to eat and may develop ulcerated areas in the eyes or be unable to open the eyes. It is important to consult a dermatologist and an ophthalmologist as quickly as possible if a patient develops Stevens-Johnson syndrome, to prevent lasting damage to the patient's eyesight. In addition, the syndrome is sometimes fatal.

A physician should be called immediately if any of these signs of a dangerous reaction occur:

Sulfa drugs may also cause dizziness. Anyone who takes sulfonamides should not drive, use machines or do anything else that might be dangerous until they have found out how these drugs affect them.

Sulfonamides may cause blood problems that can interfere with healing and lead to additional infections. Patients should try to avoid minor injuries while taking these medicines, and be especially careful not to injure the mouth when brushing or flossing the teeth or using a toothpick. They should not have dental work done until their blood is back to normal.

Sulfa medications may increase the skin's sensitivity to sunlight. Even brief exposure to sun can cause a severe sunburn or a rash. During treatment with these drugs, patients should avoid exposure to direct sunlight, especially high sun 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 protection factor (SPF) of at least 15; protect the lips with a lip balm containing sun block; and avoid the use of tanning beds, tanning booths, or sunlamps.

Babies under two months should not be given sulfonamides unless their physician has specifically ordered these drugs.

Older people may be especially sensitive to the effects of sulfonamides, increasing the chance of such unwanted side effects as severe skin problems and blood disorders. Patients who are taking water pills ( diuretics ) at the same time as sulfonamides may also be more likely to have these problems.

Special conditions

People with certain medical conditions or who are taking other medicines may have problems if they take sulfonamides. Before taking these drugs, the patient must inform the doctor about any of these conditions:

ALLERGIES. Anyone who has had unusual reactions to sulfonamides, diuretics, diabetes medicines, or glaucoma medications in the past should let his or her physician know before taking sulfonamides. The physician should also be told about any allergies to foods, dyes, preservatives, or other substances.

PREGNANCY. Some sulfonamides have been found to cause birth defects in studies of laboratory animals. The drugs' effects on human fetuses have not been studied. Pregnant women are advised, however, not to use sulfa drugs around the time of labor and delivery, because they can cause side effects in the baby. Women who are pregnant or who may become pregnant should check with their physicians about the safety of using sulfonamides during pregnancy.

LACTATION. Sulfonamides pass into breast milk and may cause liver problems, anemia, and other problems in nursing babies whose mothers take the medicine. Because of those problems, women should not breastfeed their babies when they are under treatment with sulfa drugs. Women who are breastfeeding but require treatment with sulfonamides should check with their physicians to find out how long they should stop breastfeeding.

OTHER MEDICAL CONDITIONS. People with any of the following medical problems should make sure their physicians are aware of their conditions before they take sulfonamides:

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 the drug and do not require medical treatment.

More serious side effects are not common, but may occur. If any of the following side effects occur, the patient should check with a physician immediately:

Other rare side effects may occur. Anyone who has unusual symptoms while taking sulfonamides should get in touch with his or her physician.


Sulfonamides may interact with a large number of other medicines. When an interaction occurs, the effects of one or both of the drugs may change or the risk of side effects may be greater. Anyone who takes sulfonamides should give the physician a list of all other medicines that he or she is taking. Among the drugs that may interact with sulfonamides are:

The list above does not include every drug that may interact with sulfonamides. Patients should be careful to check with a physician or pharmacist before combining sulfonamides with any other prescription or nonprescription (over-the-counter) medicine. This precaution includes herbal preparations. Some herbs, such as bearberry, parsley, dandelion leaf, and sarsaparilla, have a diuretic effect and should not be used while taking sulfa drugs. Basil, which is commonly used in cooking to flavor salad dressings, stews, and tomato recipes, is reported to affect the absorption of sulfonamides.



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Brody, T. M., J. Larner, K. P. Minneman, and H. C. Neu. Human Pharmacology: Molecular to Clinical , 2nd ed. St. Louis: Mosby Year-Book, 1995.

"Inflammatory Reactions: Erythema Multiforme." Section 10, Chapter 118 in The Merck Manual of Diagnosis and Therapy , edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.

Karch, A. M. "Lippincott's Nursing Drug Guide." Springhouse, PA: Lippincott Williams & Wilkins, 2003.

Pelletier, Kenneth R., MD. The Best Alternative Medicine , Part I, Chapter 6, "Western Herbal Medicine." New York: Simon & Schuster, 2002.

Reynolds, J. E. F., ed. Martindale: The Extra Pharmacopoeia , 31st ed. London, UK: The Pharmaceutical Press, 1996.


American Society of Health-System Pharmacists (ASHP). 7272 Wisconsin Avenue, Bethesda, MD 20814. (301) 657-3000. .

United States Food and Drug Administration (FDA). 5600 Fishers Lane, Rockville, MD 20857-0001. (888) INFO-FDA. .

other .

Nancy Ross-Flanigan Sam Uretsky, PharmD

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