Patient education: Sulfa drug allergy (The Basics)

What are sulfa drugs? — The term "sulfa drugs" is usually used to describe a specific group of antibiotics. Antibiotics are medicines used to treat bacterial infections. Sulfa antibiotics contain chemicals called "sulfonamides."

The most common sulfa antibiotic is trimethoprim-sulfamethoxazole, or "TMP-SMX." It is also called co-trimoxazole (sample brand names: Bactrim, Septra, Sulfatrim). There are others, too. They come as pills, creams, lotions, or eye drops. Examples include sulfadiazine, sulfadoxine, sulfisoxazole, and other drugs 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 sulfa antibiotics.

What is a sulfa drug allergy? — A sulfa drug allergy is a bad reaction to a sulfa antibiotic. It happens when the body's immune system reacts to the drug 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 drug as though it were an invader. But this is what happens when a person has a drug allergy.

What are the symptoms of an allergic reaction? — The most common allergic reaction to sulfa drugs is a skin rash (picture 1). It usually appears within a few days of starting a sulfa antibiotic, and goes away soon after stopping the drug. The rash can be different in different people. You might have:

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

●Bumps, spots, or red patches on your skin

●Itching

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 pain or irritation of the "mucous membranes." (This is the pink, moist tissue that lines the eyes, mouth, vagina, and other organs.) In severe cases, it can lead to organ damage.

Less commonly, sulfa drugs can cause:

●A severe immediate reaction – "Immediate" means that symptoms happen within minutes to 1 hour after taking the drug. 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, or "SJS/TEN" – This is a rare but very serious problem that affects the skin. It causes the skin to turn red or purple and peel away from the body.

Is there a test for sulfa drug allergy? — No. There is no specific test that can tell if a person is allergic to sulfa drugs. A doctor can tell if you have an allergy based on the drug 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 sulfa drug.

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 sulfa drugs 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 sulfa antibiotics – If you have had a serious reaction to sulfa drugs in the past, it is best to completely avoid these drugs. 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 have had mild reactions to sulfa drugs in the past. With test dosing, the doctor will have you take a small amount of a drug while they watch to see if you have a reaction. If you do have a reaction, they will be 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 that you are not likely to have a serious allergy.

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

What else should I know if I have a sulfa drug 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 sulfa antibiotics, do not take sulfasalazine.

●There are other medicines that contain sulfonamides but are not antibiotics. An example is "sulfonylurea" medicines used to treat diabetes. These are not likely to cause a reaction. If you need treatment with one of these medicines, your doctor or nurse can talk to you about your options.

●Tell all of your doctors and nurses about your allergy. Tell your pharmacy, too. This will help 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

●High fever

●Painful skin

●Skin blisters

●Pain and irritation of the 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)