An Overview of Sulfa Allergy

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. Such a response to sulfa is not all that uncommon. Whether caused by a true allergic reaction or other intolerance, sulfa reactions affect up to 6% of people (women more than men). This rate is similar to what is seen with reactions to other types of antibiotics, including penicillin.

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 or Bactrim (sulfamethoxazole-trimethoprim)
• Pediazole or Eryzole (erythromycin-sulfafurazole)

The symptoms and severity of a sulfa allergy can vary but usually involve the appearance of a widespread rash or hives. Occasionally, a photosensitive rash—that is, one that occurs in areas exposed to sunlight or other UV light—may develop while on the medication.

Other common symptoms may include:

• Itchy skin
• Swelling of the face, hands, feet, and tongue (known as angioedema)

Certain people may develop more severe or even life-threatening symptoms that should be treated promptly by a medical personnel.

Anaphylaxis, a life-threatening allergic reaction, may be accompanied by symptoms such as:

• Swelling of the throat or tongue and difficulty swallowing
• Difficulty breathing or rapid breathing
• Low blood pressure
• Lightheadedness or fainting
• Racing heart
• Nausea or vomiting
• Skin rash, hives, swelling, or blue-colored skin from decreased circulation
• Panic, anxiety, or confusion
• Shock

Blistering of the skin is concerning for potentially life-threatening reactions such as Stevens-Johnson syndrome and toxic epidermal necrolysis.

In some cases, serious and/or chronic consequences could result from a reaction to sulfa drugs, including:

• Liver injury or hepatitis
• Kidney damage or failure
• Pneumonia-like illness
• Vasculitis

Certain people appear to be at higher risk of sulfa allergy than others. These include people who have a suppressed immune systems for a variety of reasons, including organ transplant recipients and people with HIV/AIDS.

There's a common misconception that all sulfonamide drugs are equally likely to cause an allergic or adverse reaction in these individuals. While all sulfa drugs have the potential for this, research suggests that antibiotic sulfonamides (used to treat bacterial infections) are more likely to trigger an allergic reaction than non-antibiotic ones.

There's no validated skin or blood test available to diagnose a sulfa allergy. The diagnosis is usually made on careful review of the suspected reaction and history of current and previous medication use.

Ideally, a physician will document what specific sulfa drug was associated with what specific reaction. This is to avoid prohibiting the use of drugs that could be tolerated.

Once the drug that caused the reaction is determined, it's a good idea to keep a record of it (and any other medication allergies) handy in case you ever seek emergency care from another provider.

The first-line of treatment of a sulfa allergy is typically the termination of the suspected drug. However, in milder cases where a sulfa drug is considered essential to the treatment of an infection, an allergist or other qualified physician may supervise the administration of smaller doses and gradually increase them as the drug is better tolerated.

Anaphylaxis requires immediate epinephrine use to stop the reaction (in addition to cessation of the drug), as well as follow-up medical care.

Stevens-Johnson syndrome and toxic epidermal necrolysis are also potentially life-threatening conditions that require immediate medical evaluation; in severe cases, management in a burn unit may be required.

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 to sulfa drugs.

For many with a sulfa allergy, typically there is a low risk of reacting to non-antibiotic sulfonamides. However, topical sulfa antibiotic medications should be avoided. These include:

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

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

Since the risk of cross-reactivity to non-antibiotic sulfonamides is low, it's considered generally safe to take the following drugs: 

• Diuretics used to treat edema (fluid retention) and high blood pressure such as Bumex (bumetanide), HCTZ (hydrochlorothiazide), Thalitone (Chlorthalidone), and Lasix (furosemide)
• Diamox (acetazolamide) used to treat epilepsy, intercranial hypertension, altitude sickness, glaucoma, and heart failure
• Oral sulfonylureas-class drugs like Gynase (glyburide), (Glucotrol) glipizide, and (Amaryl) glimepiride used to treat type 2 diabetes
• Celebrex (celecoxib), a COX-2 inhibitor used to treat arthritis and pain
• Imitrex (sumatriptan), Relpax (eletriptan), Frova (frovatriptan) used to treat migraines
• Non-antibiotic eye drops like Maxitrol (neomycin/dexamethasone/polymyxin B), Genoptic (gentamicin sulfate), Isopto Atropine (atropine sulfate), and Polytrim (polymyxin B/trimethoprim)

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 adverse reaction.