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Sulfonamides (sulphonamides) are a group of man-made (synthetic) medicines that contain the sulfonamide chemical group. They may also be called sulfa drugs.

Many people use the term sulfonamide imprecisely to refer only to antibiotics that have a sulfonamide functional group in their chemical structure. However, there are several non-antibiotic sulfonamides that have been developed by exploiting observations made during the clinical evaluation of the antibiotic sulfonamides. These are used for a range of conditions such as diabetes and pain relief.

Sulfanilamide was the first sulfonamide developed in 1906, although it was not used as an antimicrobial agent until the late 1930s. Sulfonamide antimicrobials are bacteriostatic (stop bacteria from reproducing but don't necessarily kill them) and work by interfering with the synthesis of folic acid in bacteria, which is essential for nucleic acid formation and ultimately DNA and RNA. Humans obtain folic acid from their diet, but bacteria need to synthesize it. Sulfonamide antimicrobials may be combined with trimethoprim to make them bactericidal (kill bacteria), because trimethoprim acts on a different enzyme in the folic acid synthesis pathway.

Non-antibiotic sulfonamides are thought to have anti-inflammatory or immunomodulatory properties although the exact way they work in some conditions is not known.

What are sulfonamides used for?

Sulfonamides represent a diverse range of medicines with a diverse range of actions. Examples of some conditions that may be treated with sulfonamides include:

• Bacterial infections: eg, sulfamethoxazole/trimethoprim, sulfisoxazole
• Crohn’s disease: eg, sulfasalazine
• Diabetes: eg, glyburide, tolbutamide
• Fluid retention: eg, chlorothiazide, furosemide, hydrochlorothiazide
• Gout: eg, probenecid
• High blood pressure: eg, chlorothiazide, hydrochlorothiazide
• Pain and inflammation: eg, celecoxib
• Rheumatoid arthritis: eg, sulfasalazine
• Ulcerative colitis: eg, sulfasalazine.

What are the differences between sulfonamides?

Sulfonamide antibiotics have an N4 amine group in their structure which is thought to contribute to their higher incidence of allergic-type reactions. Non-antibiotic sulfonamides lack this structure.

Sulfonamide antibiotics

Oral sulfonamides are rapidly excreted and very soluble in urine and are commonly used to treat infections of the urinary tract.

Non-antibiotic sulfonamides

Many classes of drugs contain a sulfonamide structure including carbonic anhydrase inhibitors; sulfonylureas; and thiazide, thiazide-like and loop diuretics.

*Sulfasalazine also has an antibacterial effect.

Are sulfonamides safe?

Sulfonamide-containing drugs are frequently implicated in allergic reactions and nonallergic reactions.

The term “sulfa allergy” (or “sulfur allergy”) most commonly refers to an immunological response to sulfonamides, and it is a term that is often misused and misinterpreted. It should not be confused with a sulphite allergy (sulphites are substances used to preserve foods); nor with an allergy to the element sulphur or sulphates (allergic reactions to these naturally occurring substances are extremely rare). Non-antibiotic sulfonamides are thought to be less likely than antibiotic sulfonamides to cause severe allergic reactions.

Sulfonamide allergic reactions affect 1.5-3% of the population but are 10 times more likely in people with HIV. Management depends on the type and severity of the reaction. Mild reactions can be treated with drug discontinuation and antihistamine administration. More severe reactions may require topical or oral steroids and possibly hospital admission.

Sulfonamide allergies can manifest in several different ways, for example as:

• Sulfonamide drug hypersensitivity syndrome: Symptoms usually start 7 to 14 days after sulfonamide initiation and include fever and a generalized rash; internal organs may be affected
• Fixed drug eruptions: Symptoms develop within 30 minutes to 8 hours of taking the drugs and include well-defined, round or oval patches of redness and skin swelling, sometimes surmounted by a blister
• Type I immediate, IgE-mediated, true allergic response: Hives occur within 30 minutes of drug administration. Anaphylaxis is rare
• Stevens-Johnson Syndrome (SJS) / Toxic Epidermal Necrolysis (TEN): Serious, potentially fatal skin reaction that usually develops within the first week of taking the drug. Symptoms include sheets of skin detachment exposing red, oozing dermis
• Erythema nodosum: Symptoms include red, hot and painful lumps on the shins or about the knees and ankles, often associated with joint pains or fever
• Erythema multiforme: Symptoms include the appearance of skin lesions that look like targets (show three concentric zones of color). May involve any body site and the lips.

Sulfonamides may also rarely cause changes in the blood such as anemia (destruction of red blood cells), leukopenia (destruction of white blood cells), and other hematological side effects.

Renal side effects have been reported with sulfonamide use; more commonly crystals in the urine (risk is higher in people who are dehydrated) and, rarely, interstitial nephritis and tubular necrosis.

Note: Sulphites or drugs with a sulfhydryl or sulfate group in their structure (eg, captopril, morphine sulfate, heparin sulfate) do not need to be avoided by people with a sulfonamide allergy.

For a complete list of severe side effects, please refer to the individual drug monographs.

What are the side effects of sulfonamides?

Common side effects reported with sulfonamides include:

• Gastrointestinal effects (such as diarrhea, nausea, stomach upset, vomiting)
• Candidiasis (overgrowth of the yeast Candida causing conditions such as thrush) (sulfonamide antibiotics)
• Dizziness
• Fatigue
• Folate deficiency
• Headaches
• Itchy skin
• Rash.

For a complete list of side effects, please refer to the individual drug monographs.