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Sulfonamides: Drug Facts, Side Effects and Dosing

What are sulfonamides?

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.

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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 (Coumadin), 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.

Are sulfonamides safe to use while pregnant or 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 by nursing mothers because sulfamethoxazole is excreted in breast milk and can cause kernicterus.


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