Daniel More, MD, is a board-certified allergist and clinical immunologist with a background in internal medicine.

Corinne Savides Happel, MD, is a board-certified allergist and immunologist with a focus on allergic skin disorders, asthma, and other immune disorders.

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

Allergies to sulfa drugs are common. Whether caused by a true allergy or drug intolerance, sulfa reactions affect up to 6% of people. The rate is similar to responses to other types of antibiotics, including penicillin.

This article explains sulfa allergy symptoms, causes, diagnosis, and treatment. It also covers how to decipher between sulfa allergy and sulfite allergy.

Sulfa allergies are allergic-type reactions to sulfonamide drugs. If someone is sensitive to sulfa, the most common reactions manifest on the skin as rashes. However, symptoms can also be more severe.

Sulfonamides are one of the earliest developed antimicrobial drugs. They are used less frequently than in the past because safer, more effective medications have replaced them. However, they still have uses today, especially for treating certain infections, including:

The most common sulfa drug associated with sulfa allergy is sulfamethoxazole-trimethoprim (Bactrim, Septra, and others).

While they sound similar, sulfa drug allergy and sulfite allergy are not the same things. As mentioned earlier, a sulfa drug is derived from the sulfonamide molecule.

On the other hand, sulfites are compounds containing sulfuric acid used as preservatives in packaged foods and wine. They can cause reactions in some people but are not related to sulfonamides. As such, you don’t need to avoid sulfites if you have a sulfa allergy.

You'll find sulfites in the following products:

Sulfites may appear as the following on an ingredients list:

Reactions to sulfites include worsening asthma symptoms, anaphylaxis (a severe, life-threatening reaction), and hives.

The symptoms and severity of a sulfa allergy can vary from mild to life-threatening. Call a healthcare provider if you develop any signs of a sulfa allergy, even if your symptoms are mild. Seek emergency care if symptoms are severe and rapidly progressing.

In some cases, continuing a sulfa drug while having mild allergy symptoms may eventually result in severe and life-threatening effects.

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

Rashes can get worse with sun exposure.

Certain people may develop anaphylaxis, a potentially life-threatening, whole-body allergy that can lead to shock, coma, respiratory or heart failure, and death if left untreated.

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

Anaphylaxis usually occurs within minutes to several hours of a dose.

Other severe reactions can take longer to develop. For example, Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) may take up to eight weeks after exposure to a drug (usually between four and 30 days) to develop. With both of these conditions, large areas of skin will blister and peel, posing the risk of severe dehydration, shock, and death if left untreated.

Common symptoms of sulfa allergy include an itchy rash and hives. However, less commonly, more severe reactions can occur. These include anaphylaxis, Stevens-Johnson syndrome (SJS), and toxic epidermal necrolysis (TEN). Severe reactions require immediate medical attention.

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Certain people are at a higher risk of sulfa allergy than others. These include people who are immunocompromised, especially those with advanced HIV infection.

All sulfa drugs have the potential to induce an allergic reaction. However, research suggests that antibiotic sulfonamides (used to treat bacterial infections) are more likely to trigger an allergic reaction than other sulfa drugs.

Sulfa antibiotic drugs that may cause an allergic reaction include:

In addition, topical sulfa antibiotic medications can cause a reaction in hypersensitive people. These include:

Many people with a sulfa allergy can tolerate non-antibiotic sulfonamides because the risk of cross-reactivity (an allergic reaction that occurs when proteins in a substance are similar to an allergen) is lower. Even so, it is still possible to react to these medications in hypersensitive people.

In addition, people with a glucose 6 phosphate dehydrogenase (G6PD) deficiency (a lack of enough of a specific red blood cell enzyme) are at risk of developing hemolytic anemia . This type of anemia occurs when red blood cells break down too soon.

Exposure to certain blood-related things, including sulfa drugs, can trigger hemolytic anemia in people with G6PD. This reaction is entirely different from an allergy.

Immunocompromised people, particularly those with advanced HIV, are most at-risk of a sulfa drug allergy. Antibiotic sulfa medications pose the most significant risk of a reaction.

However, topical medications and non-antimicrobials can also cause reactions. In addition, sulfa drugs can trigger hemolytic anemia in those with G6PD deficiency.

While healthcare providers can diagnose some drug allergies through allergy testing, that is not the case for sulfa drugs. Instead, a diagnosis usually relies on carefully examining symptoms and a review of your current and previous medication use.

In some cases, an allergist (an allergy, asthma, and immunology specialist) may recommend an oral drug challenge. This test involves taking the medication suspected of causing a reaction under the supervision of a healthcare provider. However, this may be too dangerous if your past response was severe.

Some experts believe this type of testing should be the standard for suspected sulfa drug allergies in immunocompromised people due to their high antibiotic usage. However, this is not yet the norm.

The first-line treatment of a sulfa allergy is stopping the suspected drug. However, in milder cases where a sulfa drug is essential to treating an infection, your healthcare provider may supervise smaller doses, gradually increasing the amount as tolerated. This process is called allergy desensitization.

Treatment for more severe reactions might include:

SJS and TEN may also require burn center care.

This photo contains content that some people may find graphic or disturbing.

Always check with a healthcare provider before starting a new medication if you have a known sulfa allergy. This precaution is especially crucial if you had a previous severe reaction to sulfa drugs.

Ideally, a healthcare provider will document the specific sulfa drug and the specific reaction rather than listing an allergy to a whole class of drugs. Doing so helps prevent future use, even if sold under a different trade name.

Once you identify the drug that caused the reaction, it’s essential to record it. You may even choose to wear a medical bracelet identifying your allergy. In addition, some people who have had severe reactions keep documentation on them to avoid receiving the drug in the future.

Sulfa allergies are an allergy to a group of medications containing sulfonamides. It is not the same as a sulfite allergy, an allergy to a food preservative. The most common symptom is an itchy rash. However, more severe reactions, including anaphylaxis, Stevens-Johnson syndrome (SJS), and toxic epidermal necrolysis (TEN), can also occur.

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 healthcare provider 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 healthcare provider to prescribe a substitute for you that is less likely to cause an adverse reaction.

Usually, a healthcare provider diagnoses a sulfa allergy based on symptoms that occur while taking a sulfa drug. There isn't a blood or skin test that can definitively diagnose a sulfa allergy, but sometimes an allergist may recommend an oral drug challenge to confirm an allergy. However, this is probably too dangerous if your past reaction was severe.

Researchers believe that sulfa reactions occur in up to 6% of people.

Sulfa drugs are an umbrella term for medications that contain sulfonamides. These include some antibiotics and nonantimicrobial medications.

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