Sulfa Allergy Symptoms and Risks

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's a common misconception that all sulfonamide drugs have the potential to cause allergy, but this isn't 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 3 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 those who, for various reasons, metabolize the drug slowly or have a suppressed immune system (such as organ transplant recipients and people with HIV/AIDS).

The symptoms and severity of a sulfa allergy can vary but usually 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 rare, 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's 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's 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 of a sulfonamide drug.

The first-line of treatment of a sulfa allergy is typically the termination of the suspected drug.

Emergency medical treatment may be required in people 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.

People with a known sulfa allergy should always check with their doctor before 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. These include:

• 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.

Keep in mind that the risk of cross-reactivity to non-antibiotic sulfonamides is low. This means that it's 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

People will often mistake a sulfa allergy for a sulfite allergy. Sulfites are preservatives found in foods and medications. These include:

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

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

This also applies to sulfates, which are drugs containing sulfuric acid. 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. That's why it's important to tell your doctor about any prior reaction you may have had to a sulfa medication (or any other drug for that matter). Sharing that information will make it easier for your doctor to prescribe a substitute that's less likely to cause an allergy.