Patient education: Sulfonamide allergy (The Basics)

What are sulfonamides? — 

This term is usually used to describe a specific group of antibiotics. More commonly, these are called "sulfa drugs."

Antibiotics are medicines used to treat bacterial infections. "Sulfonamides" are chemicals found in certain antibiotics.

The most common sulfonamide antibiotic is trimethoprim-sulfamethoxazole, or "TMP-SMX." It is also called "co-trimoxazole" (sample brand names: Bactrim, Septra, Sulfatrim). There are others, too, but they are not used very much. They come as pills, creams, lotions, or eye drops. Examples include sulfadiazine, sulfadoxine, sulfisoxazole, and other medicines that start with "sulf."

There are non-antibiotic medicines that contain sulfonamides, too. Most of these can be used even in people with an allergy to sulfonamide antibiotics.

What is a sulfonamide allergy? — 

This is a bad reaction to a sulfonamide antibiotic. It happens when the body's immune system reacts to the medicine as if it were a dangerous invader and tries to fight it off.

The immune system's job is to fight off infection. Normally, it should not react to a medicine as though it were an invader. But this is what happens when a person has a medicine (drug) allergy.

This type of allergic reaction is also called "sulfonamide hypersensitivity."

What are the symptoms of an allergic reaction? — 

The most common allergic reaction to sulfonamides is a skin rash (picture 1). It usually appears within a few days of starting a sulfonamide antibiotic, and goes away soon after stopping the medicine. The rash can be different in different people. You might have:

●Bumps, spots, or red patches on your skin

●Itching

●Hives, which are raised patches of skin that are usually very itchy (picture 2)

In some cases, a rash is a sign of a more serious reaction. When this happens, a person might have a high fever, feel very tired, or have nausea. In severe cases, it can lead to organ damage.

Less commonly, sulfonamides can cause:

●A severe immediate reaction – "Immediate" means symptoms happen within minutes to 1 hour after taking the medicine. They can include hives, swelling, wheezing, nausea or vomiting, and dizziness. If not treated, this can turn into a life-threatening allergic reaction called "anaphylaxis."

●Stevens-Johnson syndrome and toxic epidermal necrolysis ("SJS/TEN") – This is a rare but very serious reaction. It causes the skin to turn red or purple, blister, and peel away from the body. A person can also have pain or irritation of the "mucous membranes." (These are the pink, moist tissues that line the eyes, mouth, vagina, and other organs.)

Is there a test for sulfonamide allergy? — 

No. There is no specific test that can tell if a person is allergic to sulfonamides. A doctor can tell if you have an allergy based on the medicine you took and what symptoms you had.

How are allergic reactions treated? — 

The first step is to stop taking the medicine that caused the reaction.

If you got a rash without other symptoms, this usually goes away on its own once you stop taking the medicine.

Doctors can treat severe immediate reactions with a medicine called "epinephrine." If you had a mild reaction, like hives or itching, an "antihistamine" medicine might help.

How can I prevent an allergic reaction? — 

If you ever need treatment with sulfonamides in the future, your doctor can work with you to decide what to do. They might also send you to an allergist (allergy doctor).

In general, the options are:

●Avoid all sulfonamide antibiotics – If you had a serious reaction to a sulfonamide antibiotic in the past, it is best to completely avoid these medicines. Most of the time, other antibiotics can be used instead.

●Test dosing – This is also called a "drug challenge" test. Your doctor might recommend this if you had only mild reactions to sulfonamides in the past. With test dosing, you take a small amount of the medicine while the doctor watches to see if you have a reaction. If you do, they are ready to treat it right away. If you do not, they can slowly increase the dose. Test dosing is usually only done if the doctor thinks you are not likely to have a serious allergy.

●Desensitization – Certain infections can only be treated with sulfonamide antibiotics. If you get one of these infections and you need treatment, your doctor might recommend "desensitization." This involves giving you the medicine in a controlled way, starting with very small amounts. It can cause your body to tolerate the medicine for a short time without having a reaction.

What else should I know if I have a sulfonamide allergy?

●There are other "sulfa drugs" that are not antibiotics. One of these, sulfasalazine, is used to treat arthritis and colitis. If you have a serious allergy to sulfonamide antibiotics, do not take sulfasalazine.

●There are other medicines that contain sulfonamides but are not antibiotics. Examples are diuretics (or "water pills") that help the body get rid of extra salt and fluid. Diuretics are not likely to cause a reaction in a person with an allergy to sulfonamide antibiotics. If you need treatment with one of these medicines, your doctor or nurse can talk to you about your options.

●Tell all your doctors and nurses about your allergy. Tell your pharmacy, too. This helps you stay safe if you need antibiotics or other medicines in the future.

When should I call the doctor? — 

That depends on what kind of reaction you have.

Call for an ambulance (in the US and Canada, call 9-1-1) if you start a new medicine and have any of these symptoms:

●Wheezing or trouble breathing

●Chest tightness or pain

●Passing out, or feeling like you might pass out

●Swelling of your face, lips, tongue, or throat that makes it hard to breathe

Call your doctor or nurse if you start a new medicine and have any of these symptoms:

●Hives

●Swelling of the face

●Fever

●Painful skin

●Skin blisters

●Pain and irritation of mucous membranes

More on this topic

Patient education: Drug allergy (The Basics)
Patient education: Allergy skin testing (The Basics)
Patient education: Anaphylaxis (The Basics)
Patient education: Side effects from medicines (The Basics)
Patient education: Stevens-Johnson syndrome and toxic epidermal necrolysis (The Basics)
Patient education: What you should know about antibiotics (The Basics)

Patient education: Anaphylaxis symptoms and diagnosis (Beyond the Basics)
Patient education: Anaphylaxis treatment and prevention of recurrences (Beyond the Basics)