Medically Reviewed on 4/26/2023

What are sulfonamides, and what are they used for?

Sulfonamides (sulfa drugs) are drugs that are derived from sulfanilamide, a sulfur-containing chemical. Most sulfonamides are antibiotics, but some are prescribed for treating ulcerative colitis. Sulfonamide antibiotics work by disrupting the production of dihydrofolic acid, a form of folic acid that bacteria and human cells use for producing proteins.

What are the side effects of sulfonamides?

Sulfonamides may cause:

Sulfonamides should be stopped at the first appearance of a skin rash before the rash becomes severe.

Serious rashes include:

  • Stevens-Johnson syndrome, which includes symptoms like:
    • aching joints,
    • aching muscles,
    • redness,
    • blistering, and
    • peeling of the skin
  • Toxic epidermal necrolysis, which includes symptoms like:
    • difficulty in swallowing,
    • peeling,
    • redness,
    • loosening, and
    • blistering of the skin.

Sulfonamides also may cause sensitivity to the sun that leads to extensive sunburn after exposure to sunlight (photosensitivity). Patients receiving sulfonamides should avoid excessive exposure to sunlight and should wear sunscreen.

Other rare side effects include liver damage, low white blood cell count (leucopenia), low platelet count (thrombocytopenia), and anemia. Formation of urinary crystals which may damage the kidney and may cause blood. Adequate hydration is needed to prevent the formation of urinary crystals.

What are examples of sulfonamides available in the US?

Examples of sulfonamides include:

Many of these drugs are available only in generic forms.

What drugs interact with sulfonamides?

Sulfonamides can increase the blood-thinning effects of warfarin, possibly leading to abnormal bleeding.

The increased metabolism (break-down and elimination) of cyclosporine by the liver caused by sulfonamides (reduces the effectiveness of cyclosporine and can add to the kidney damage caused by cyclosporine.

All sulfonamides can crystallize in the urine when the urine is acidic. Since methenamine (Hiprex, Urex, Mandelamine) causes acidic urine, it should not be used with sulfonamides.

Blood levels of digoxin may increase blood levels of digoxin (Lanoxin) and possibly lead to serious toxic effects.

Anemia, due to a reduction in folic acid, can occur in persons receiving sulfonamides in combination with divalproex, valproic acid (Depakote, Depakote ER, Depakene, Depacon, Stavzor), methotrexate (Rheumatrex, Trexall), pyrimethamine, triamterene, or trimetrexate.

Increased blood levels of potassium may occur when sulfamethoxazole/trimethoprim is combined with angiotensin converting enzyme (ACE) inhibitors.


Bowel regularity means a bowel movement every day. See Answer

What formulations of sulfonamides are available?

Sulfonamides are available as tablets, injections, and oral solutions.

Pregnancy and breastfeeding

Use of sulfonamides may cause bilirubin to be displaced from proteins in the infant's blood. Displacement of bilirubin can lead to jaundice and a dangerous condition called kernicterus in the infant. For this reason, sulfamethoxazole/ trimethoprim should not be used near term (late in pregnancy) among women.

Sulfonamides (for example, sulfamethoxazole/trimethoprim) should not be used while nursing because sulfamethoxazole is excreted in breast milk and can cause kernicterus.


Sulfonamides are a class of drugs from a sulfur-containing chemical (sulfanilamide). Examples of sulfonamides include sulfamethoxazole/trimethoprim (Bactrim, Bactrim DS, Co-trimoxazole, Septra, Septra DS, Cotrim, SMZ-TMP, SMZ-TMP DS, Sulfatrim); sulfasalazine (Azulfidine, Sulfazine); and sulfisoxazole (Truxazole, Gantrisin). Some of these drugs are available only in generic forms. Side effects of sulfonamides may include dizziness, lethargy, diarrhea, nausea, vomiting, headache, serious skin rashes and anorexia. Sulfonamides are not recommended for women who are pregnant or breastfeeding. Side effects and drug interactions should be reviewed prior to taking this medication.

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Medically Reviewed on 4/26/2023
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