Sulfa allergy is an adverse drug reaction, such as a rash, to sulfonamides , a class of drugs that includes antibiotics and other drugs.
Allergies to sulfa drugs are common. Whether caused by a true allergy or drug intolerance, sulfa reactions happen at a rate similar to other types of antibiotics, including penicillin. Methylation
Sulfonamides treat conditions such as urinary tract infections (UTIs), inflammatory bowel disease (IBD), infections of the skin, eye, and vagina, burns, and malaria.
This article explains sulfa allergy symptoms, causes, diagnosis, and treatment. It also covers how to tell the difference between sulfa allergy and sulfite allergy.
Sulfa allergies are allergic 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).
Researchers believe that sulfa reactions occur in 3% to 8% of people.
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:
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.
Anaphylaxis can occur within seconds to minutes, but sometimes symptoms may emerge two hours after exposure.
Seek emergency care if you experience the following signs of anaphylaxis:
Stevens-Johnson syndrome (SJS), a rare but serious drug reaction, can occur with sulfa medications. With SJS and toxic epidermal necrolysis (TEN), large areas of skin blister and peel, posing the risk of severe dehydration, shock, and death if left untreated.
Unlike anaphylaxis, SJS/TEN takes time for symptoms to develop. Painful blisters typically appear between four and 30 days after starting a medication. It can take as long as eight weeks of taking a sulfa drug or other medication before the unmistakable rash is evident.
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All sulfa drugs have the potential to induce an allergic reaction, but some are more likely to cause a reaction.
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.
With a sulfa allergy, the immune system has an abnormal response against the drug, which then causes symptoms.
Certain people are at a higher risk of sulfa allergy than others. These include people who are immunocompromised and those with advanced HIV infection.
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 while taking a sulfa drug and reviewing 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 for a sulfa allergy is stopping the suspected drug. However, in cases of suspected anaphylaxis, drug desensitizations can be performed in which small amounts of medication are slowly titrated upwards until a full dose can be tolerated.
For safety, this must be done under physician supervision with appropriate staffing and supplies to treat severe anaphylaxis. As long as people stay on the medication(s) that they have been desensitized to, they should not react. If they stop the medication, they would need to undergo another desensitization prior to receiving the medication again.
Stevens-Johnson syndrome and toxic epidermal necrolysis reactions require immediate medical attention and hospitalization possibly in a burn care center.
Treatment for more severe reactions might include:
Letting your medical team know the specific sulfa medication you've reacted to and adding this to your medical record can help providers know how to best treat you in the future.
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If you have a known sulfa allergy, take these steps to prevent future reactions:
While they sound similar, sulfa drug allergy and sulfite allergy are not the same thing. As mentioned above, 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.
Sulfites are found in dried and pickled foods, shrimp, condiments, beer, wine, and processed foods. 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.
A sulfa allergy is an allergy to a group of medications containing sulfonamides. Sulfonamides are found in antibiotics, burn creams, vaginal suppositories, eye drops, and other medications.
The most common sulfa allergy symptom is an itchy rash. However, potentially fatal reactions, including anaphylaxis, Stevens-Johnson syndrome (SJS), and toxic epidermal necrolysis (TEN), can also occur.
A sulfa allergy is not the same as a sulfite allergy, which is an allergy to a food preservative.
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By Daniel More, MD Daniel More, MD, is a board-certified allergist and clinical immunologist. He is an assistant clinical professor at the University of California, San Francisco School of Medicine and formerly practiced at Central Coast Allergy and Asthma in Salinas, California.
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