Sulfa Allergy: Symptoms, Causes, Diagnosis, and Treatment

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 allergy or drug intolerance, sulfa reactions affect up to 6% of all people (women more than men). The rate is similar to what is seen with reactions to other types of antibiotics, including penicillin.

Common symptoms of a sulfa allergy — Illustration by Brianna Gilmartin, Verywell

Two of the more common sulfa drugs associated with sulfa allergy are:

Sulfamethoxazole-trimethoprim (marketed under the brand names Bactrim, Septra, and others)
Erythromycin-sulfafurazole (marketed under the brand names Eryzole, Pediazole, and others)

Symptoms

The symptoms and severity of a sulfa allergy can vary from mild to life-threatening. Call your doctor if you develop symptoms of a sulfa allergy, however mild, and seek emergency care if symptoms are severe and rapidly progressing.

In some cases, continuing a sulfa drug while having mild symptoms may cause those mild symptoms to become severe and life-threatening.

Common

Common symptoms of sulfa allergy are often limited to the skin and may include:

Pruritus (itchy skin)
Urticaria (hives)
Generalized maculopapular rash (a widespread rash with flat and raised lesions)
Angioedema (swelling of the face, hands, and other tissues)
Photosensitive rash (one that occurs in areas of ultraviolet light exposure)

Severe

Certain people may develop more severe or even life-threatening symptoms that require emergency medical attention. Chief among these is anaphylaxis, a potentially life-threatening, whole-body allergy that can lead to shock, coma, respiratory or heart failure, and death if left untreated.

When to Call 911

Seek emergency care if you experience the following signs of anaphylaxis:

Swelling of the throat or tongue
Difficulty swallowing
Difficulty breathing or rapid breathing
Wheezing
Lightheadedness or fainting
Racing heart or irregular heartbeat
Nausea or vomiting
Severe rash or hives
Blue-colored skin
A feeling of impending doom

Anaphylaxis tends to occur within minutes to several hours of a dose.

Other reactions take longer to develop. This includes reactions know as Stevens-Johnson syndrome and toxic epidermal necrolysis in which large areas of skin will blister and delaminate, posing the risk of severe dehydration, shock, and death if left untreated.

As opposed to anaphylaxis, SJS and TEN typically develop within 8 weeks of exposure to a drug (usually between 4 and 30 days).

SJS on face — Stevens-Johnson syndrome on the face. DermNet / CC BY-NC-ND

Causes

Certain people appear to be at higher risk of sulfa allergy than others. These include people who are severely immunocompromised, including organ transplant recipients and people with advanced HIV infection.

There's a common misconception that all sulfonamide drugs are equally likely to cause an allergic or adverse reaction. 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.

Diagnosis

There are no skin or blood tests available to diagnose a sulfa allergy. The diagnosis is usually based on a careful examination of symptoms as well as a review of your current and previous medication use.

Ideally, a physician will document what specific sulfa drug was associated with what specific reaction. This ensures that the drug is avoided in the future even if it is sold under a different trade name.

Once the drug that caused the reaction is determined, it's a good idea to keep a record of it (and even wear a medical bracelet advising of the concern) in the event of a future incident.

Treatment

The first-line treatment of a sulfa allergy is the termination of the suspected drug. However, in milder cases where a sulfa drug is considered essential to the treatment of an infection, your doctor and/or allergist may supervise the administration of smaller doses, gradually increasing the dose as tolerated. This is referred to as allergy desensitization.

Anaphylaxis requires an immediate epinephrine injection to halt the reaction, followed by supportive care and non-inflammatory medications such as corticosteroids.

SJS and TEN also require emergency medical treatment, including pain medications, corticosteroids, and intravenous hydration. Severe cases often require the care of a burn center.

SJS on trunk — Stevens-Johnson syndrome on backside. DermNet / CC BY-NC-ND

Prevention

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, there typically is a low risk of reacting to non-antibiotic sulfonamides.

However, topical sulfa antibiotic medications should be avoided if you are known to be hypersensitive. 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 Crohn's disease, ulcerative colitis, and rheumatoid arthritis should be avoided.

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

Celebrex (celecoxib), a COX-2 inhibitor used to treat arthritis
Diamox (acetazolamide) used to treat epilepsy, intracranial hypertension, glaucoma, and heart failure
Diuretics like Bumex (bumetanide), HCTZ (hydrochlorothiazide), Lasix (furosemide), and Thalitone (chlorthalidone),
Migraine medications like Frova (frovatriptan), Imitrex (sumatriptan), and Relpax (eletriptan)
Non-antibiotic eye drops like Genoptic (gentamicin sulfate), Isopto Atropine (atropine sulfate), Maxitrol (neomycin + dexamethasone + polymyxin B), and Polytrim (polymyxin B + trimethoprim)
Oral sulfonylureas-class drugs like Amaryl (glimepiride), Glynase (glyburide), and (Glucotrol) glipizide used to treat type 2 diabetes

Sulfonamides vs. Sulfates

Sulfates are compounds containing sulfuric acid that are used as preservatives in packaged foods and wine. They can cause reactions in some people but are not related in any way to sulfonamides. As such, you don't need to avoid sulfates if you have a sulfa allergy.

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 is less likely to cause an adverse reaction.