Sulfa Allergy Symptoms and Risks

Debunking Common Myths About Sulfa Drug Reactions

john/Wikimedia Commons

Sulfa allergy is a term used to describe an adverse drug reaction to sulfonamides, a class of drugs that includes both antibiotics and non-antibiotics.

There is a common misconception that all sulfonamide drugs have the potential to cause allergy, but this is not entirely true. Antibiotic sulfonamides (used to treat bacterial infections) are more likely to trigger an allergic reaction than nonantibiotic ones.

While all drugs have the potential to cause an allergy, non-antibiotic sulfonamides are less likely sources.


In the majority of cases, a person with a sulfa allergy will have experienced an allergic reaction to one or more of the following antibiotic drugs:

  • Septra (sulfamethoxazole and trimethoprim)
  • Bactrim (sulfamethoxazole and trimethoprim)
  • Pediazole (erythromycin and sulfafurazole)

These reactions are not all that uncommon and affect around three percent of all people. This is a rate similar to what is seen with other types of antibiotics, including penicillin.

Certain people appear to be at higher risk of sulfa allergy than others. These include individuals who, for various reasons, metabolize the drug slowly or have a suppressed immune system (such organ transplant recipients and people with HIV).


The symptoms and severity of a sulfa allergy can vary but typically involve the appearance of a widespread rash.

Hives and light sensitivity (photosensitivity) are also common.

In more severe cases, the outbreak of rash may be accompanied by the swelling of the face, hands, feet, and tongue (known as angioedema). This is often the precursor to potentially life-threatening, all-body reactions known as Stevens-Johnson syndrome and toxic epidermal necrolysis.

Both conditions are characterized by the severe blistering of the skin and rapid tissue death.

Other serious manifestations of a sulfa allergy include:

  • Liver injury, characterized by the development of hepatitis
  • Kidney damage or failure
  • Pneumonia-like respiratory illness
  • Vasculitis, the constriction of blood vessels and blood flow

It is important to take every drug-related rash seriously no matter how mild it may be. In some cases, the ongoing use of a sulfa drug can transform a mild rash into a life-threatening event.


There is no blood or skin test available to diagnose a sulfa allergy. The diagnosis is made entirely on the presentation of symptoms coinciding with the use a sulfonamide drug.


Typically speaking, the first-line of treatment of a sulfa allergy is the termination of the suspected drug.

Emergency medical treatment may be required in persons with symptoms of Stevens-Johnson syndrome or toxic epidermal necrolysis. This would typically involve hospitalization, intravenous fluids, and the same interventions used when dealing with a severe burn.

In milder cases where a sulfa drug is considered essential to the treatment of an infection, smaller doses may be given and gradually increased as the drug is better tolerated.

This often requires the supervision of an allergist experienced in toxic events.

Medications to Avoid

Persons with a known sulfa allergy should always check with their doctor before to starting a new medication. This is especially true for those who have had a previous severe reaction.

In addition to oral antibiotics, topical sulfonamides should be avoided including:

  • Sulfacetamide eye drops, shampoos, or creams
  • Silver sulfadiazine ointments used to treat burns
  • Sulfanilamide vaginal preparations

Similarly, the oral drug Azulfidine (sulfasalazine), used to treat inflammatory bowel disease and rheumatoid arthritis, should be avoided.

On the other hand, the risk of cross-reactivity to non-antibiotic sulfonamides is low. This means that it is generally safe to take the following drugs:

  • Diuretics (water pills) such as HCTZ (hydrochlorothiazide) and Lasix (furosemide)
  • Oral sulfonylureas-class drugs used to treat diabetes
  • Celebrex (celecoxib), a COX-2 inhibitor used to treat arthritis and pain

Sulfite and Sulfate Allergies

People will often mistake a sulfa allergy for a sulfite allergy, the latter of which is related to preservatives found in foods and medications. These include such preservative agents as:

  • Sodium sulfite
  • Sodium bisulfite
  • Sodium metabisulfite
  • Potassium bisulfite
  • Potassium metabisulfite

While sulfites can cause ​allergy, there is no direct relationship between a sulfa and sulfite allergy. As such, a person with a sulfa allergy doesn't have to avoid sulfites (or vice versa).

The same applies to drugs containing sulfuric acid known as sulfates. As with sulfites, sulfates may cause allergy, but the drugs are in no way related to sulfonamides or sulfa-allergy risk. These include such medications as:

  • Albuterol sulfate used to treat bronchial spasms
  • Iron sulfate used to treat iron-deficiency anemia
  • Chondroitin sulfate used to treat osteoarthritis
  • Codeine sulfate, an opioid drug used for pain relief

A Word From Verywell

The nuances of a sulfa allergy can be tricky to tease out, even for some healthcare providers. This is why it is important to tell your doctor about any prior reaction you may have had to a sulfa medication (or any other drug for that matter).

By doing so, your doctor will be better able to find a substitute less likely to cause allergy.

View Article Sources
  • Joint Task Force on Practice Parameters, et al. "Drug allergy: an updated practice parameter." Ann Allergy Asthma Immunol. 2010; 105(4):259-73. DOI: 10.1016/j.anai.2010.08.002.
  • Wulf, N. and Matuszewski, K. "Sulfonamide cross-reactivity: is there evidence to support broad cross-allergenicity?" Am J Health Syst Pharm. 2013 Sep 1;70(17):1483-94. DOI: 10.2146/ajhp120291.
  • Zawodniak, A.; Lochmatter, P.; Beeler, A. et al. "Cross-reactivity in drug hypersensitivity reactions to sulfasalazine and sulfamethoxazole. " Int Arch Allergy Immunol. 2010;153(2):152-6. DOI: 10.1159/000312632.