Key takeaways:
Antibiotics containing chemicals called sulfonamides, or “sulfa drugs,” can trigger an allergic reaction if you have a sulfa allergy.
Sulfa allergies can range from mild to severe. Some common allergy symptoms include skin rashes, hives, and fever. More severe ones include skin blisters or difficulty breathing.
If you're allergic to a sulfonamide antibiotic such as Bactrim (sulfamethoxazole / trimethoprim), you may be able to take other types of sulfa drugs without a reaction.
Allergic reactions to sulfa drugs can seem intimidating. But if you have a sulfa allergy, millions of people are right there with you.
Antibiotic medications such as Bactrim (sulfamethoxazole / trimethoprim) are some of the most common offenders for sulfa allergies. But there’s a frequent misconception about sulfa allergies. It’s often thought that if you’re allergic to one sulfa drug, you’re allergic to all of them. This isn’t always true. If you have a sulfa allergy, there are still certain medications that may be safe for you to take.
Below, we’ll explain what a sulfa allergy is and how to know if you have one. We’ll also discuss which medications may still be safe to take if you have an allergy to sulfa antibiotics.
What is a sulfa allergy?
A sulfa allergy is an allergic reaction that happens when your immune system reacts negatively to a sulfa drug. More specifically, when your immune system negatively reacts to the “sulfonamide" portion of a medication. The response can trigger your body to release histamine, which is a natural chemical your body makes when you have an allergic reaction.
Common sulfa allergy symptoms
If you have a sulfa allergy, the specific symptoms you’ll experience can vary. Common symptoms of a sulfa allergy include:
Hives
Fever
Itchy skin
Sensitivity to sunlight
Swelling of the hands or feet
Nausea, vomiting, and diarrhea — also common medication side effects unrelated to an allergic reaction
If you experience any symptoms like these after starting a sulfa drug, contact your healthcare team right away. They’ll help you monitor your symptoms and strategize how to manage them. In many cases, they’ll also tell you to stop taking the sulfa drug.
Serious sulfa allergy symptoms
Some people may develop anaphylaxis after taking a sulfa drug, a rare but potentially life-threatening allergic reaction. Anaphylaxis can occur within minutes to hours of taking a sulfa-containing medication.
Other serious allergic symptoms may include:
Difficulty breathing or swallowing
Swelling of throat or tongue
Irregular heartbeat
Unusual bleeding
Blue-colored skin
Separately, there have also been rare reports of Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). Both of these conditions can cause blisters and peeling of the skin, mouth, or genitals.
If you develop signs or symptoms of anaphylaxis, SJS, or TEN, call 911 immediately or go to the nearest ER.
Sulfa vs. sulfite allergy
The terms sulfa and sulfite sound similar, but they’re not the same thing. They’re chemically different. Having an allergy to one doesn’t mean you’ll react to the other.
Lived experiences: Three people share what it feels like to have an allergic reaction to a sulfa antibiotic called Bactrim (sulfamethoxazole / trimethoprim).
Have a UTI and can’t take Bactrim? Not to worry. There are several other urinary tract infection (UTI) antibiotics to choose from.
Drug allergies 101: Learn how you and your healthcare team can work together to prevent, treat, and diagnose allergies to a medication.
A sulfa allergy is a reaction to certain medications that contain sulfonamides, such as some antibiotics. A sulfite allergy, on the other hand, is a reaction to preservatives commonly found in foods and drinks, such as wine, dried fruit, and pre-packaged goods.
Reactions to sulfa drugs often include rashes and fever. Serious symptoms, such as difficulty breathing or low blood pressure, can occur, but they’re not as common. Sulfite reactions are more likely to cause asthma-like symptoms, hives, or other allergic responses.
These are two distinct types of allergies, so diagnosing them involves different tests and evaluations. Knowing the difference can help you avoid unnecessary dietary or medication restrictions and to help you get the right care when you need it.
What are sulfa drugs?
Sulfa drugs contain a chemical known as sulfonamide. These often include antibiotics used to prevent and treat bacterial infections. These medications come in formulations such as oral pills or liquids, topical products, and eye drops. One of the most common sulfa antibiotics is Bactrim.
In addition to sulfa antibiotics, some nonantibiotic medications contain sulfonamides too. Top examples at the pharmacy are:
Celecoxib (Celebrex)
Sumatriptan (Imitrex)
Sulfasalazine (Azulfidine)
Several diuretics (water pills), such as furosemide (Lasix) and hydrochlorothiazide (Microzide)
Certain diabetes medications, such as glyburide (Glynase, DiaBeta) and glimepiride
Sulfa drugs like these can treat several health conditions, such as:
Seborrheic dermatitis
Urinary tract infections (UTIs)
Rosacea
Wounds
Eye infections
Crohn’s disease
Ulcerative colitis
Rheumatoid arthritis
Diabetes
High blood pressure
Pneumonia
Leprosy
Migraines
Should you avoid certain antibiotics if you have a sulfa allergy?
Yes. You should avoid certain antibiotics if you’ve had an allergic reaction in the past to a sulfa drug.
The following sulfonamide antibiotics can cause a reaction if you have a sulfa allergy:
Bactrim (or Septra, another branded version of sulfamethoxazole / trimethoprim)
Silver sulfadiazine (Silvadene, SSD)
Dapsone (Aczone)
What if you need a sulfa drug?
If you have a sulfa allergy and need an antibiotic, your healthcare professional will likely give you a non-sulfonamide antibiotic. But there are certain situations where sulfa drugs such as Bactrim are still go-to choices — even if you’ve had a minor reaction in the past. One example would be for a certain type of pneumonia prevention in people with weakened immune systems.
In other cases, a sulfa rechallenge or desensitization might be considered. This is especially true if no other medications are good options. A rechallenge involves restarting the sulfa drug that was stopped. Desensitization is a procedure where you’re slowly given the drug in higher amounts until you can tolerate the needed dose.
Both of these procedures are typically done in a clinic or healthcare setting so immediate medical care is available if problems arise.
However, there are other medications you may be able to take. This is due to the chemistry of how they’re made.
Sulfa drugs to consider for people with Bactrim allergies
Many people with sulfa allergies can tolerate nonantibiotic sulfonamide medications. Except for sulfasalazine, nonantibiotic sulfonamides show a low risk of evidence of cross-reactivity. This means they shouldn't trigger an allergic reaction if you have an allergy to sulfa antibiotics.
If you’ve had an allergic reaction to an antibiotic like Bactrim, but it wasn’t severe, the sulfa-based drugs in the table below are possibilities for you to consider taking.
Important: Before you take any of these medications, make sure to check in with your healthcare team. They can tell you if these medications are truly safe for you to take.
Medication type | Medication name |
Darunavir Fosamprenavir Tipranavir (Aptivus) | |
Acetazolamide Brinzolamide (Azopt) Dorzolamide Methazolamide | |
COX-2 selective nonsteroidal anti-inflammatory drugs (NSAIDs) | Celecoxib |
Diuretics (“water pills”) | Bumetanide Chlorothiazide Chlorthalidone (Thalitone) Furosemide Hydrochlorothiazide Indapamide Metolazone Torsemide |
Glimepiride Glipizide (Glucotrol XL) Glyburide Tolbutamide | |
Almotriptan Eletriptan (Relpax) Frovatriptan (Frova) Naratriptan Rizatriptan (Maxalt, Maxalt-MLT) Sumatriptan Zolmitriptan (Zomig) | |
Blood glucose-raising medication | Diazoxide (Proglycem) |
Antigout | Probenecid |
Tamsulosin (Flomax) | |
Zonisamide (Zonegran) |
How to prevent a sulfa allergic reaction
If you’ve had an allergic reaction to a sulfa drug in the past, speak with your healthcare team about medications you should avoid and medications you can try. They’re your best resource for navigating your options and preventing future reactions.
Some precautions can go a long way for your safety, too:
Don’t take the medication that you’re allergic to.
Inform all of your healthcare professionals and pharmacies about your allergy and make sure they document it in your medical record.
Document the medication you’re allergic to and the reaction you had on your current medication list.
Carry an EpiPen (epinephrine) in the case of an emergency, especially if your previous reaction was severe.
Wear a medical bracelet or carry a medical ID card that states your allergy.
Frequently asked questions
Amoxicillin (Amoxil) isn’t a sulfa drug. It’s part of the penicillin family of antibiotics. It doesn’t contain any sulfonamide ingredients. People with a sulfa allergy don’t need to worry about avoiding amoxicillin since it’s chemically different. However, if you’re allergic to penicillin, you should avoid amoxicillin too.
The most common sulfa-based UTI medication is sulfamethoxazole / trimethoprim, better known as Bactrim or Septra. This combination is frequently prescribed for UTIs. Another example is sulfadiazine, but it’s much less common overall.
Amoxicillin (Amoxil) isn’t a sulfa drug. It’s part of the penicillin family of antibiotics. It doesn’t contain any sulfonamide ingredients. People with a sulfa allergy don’t need to worry about avoiding amoxicillin since it’s chemically different. However, if you’re allergic to penicillin, you should avoid amoxicillin too.
The most common sulfa-based UTI medication is sulfamethoxazole / trimethoprim, better known as Bactrim or Septra. This combination is frequently prescribed for UTIs. Another example is sulfadiazine, but it’s much less common overall.
The bottom line
Many people have allergies to sulfa medications. Sulfa drugs, such as Bactrim (sulfamethoxazole / trimethoprim), can cause symptoms such as skin rashes, hives, and fever. More serious reactions, including blistering skin rashes or anaphylaxis, are also possible.
Not all sulfa drugs work the same way, though. If you have a sulfa allergy, some sulfa drugs may still be safe for you to take. Your healthcare team can tell you if you can still take medications such as celecoxib (Celebrex), sumatriptan (Imitrex), furosemide (Lasix), and more.
Why trust our experts?
References
American Academy of Allergy Asthma & Immunology. (2024). Sulfite and Sulfa Drugs.
ClinicalInfo.HIV.gov. (n.d.). Desensitization.
ClinicalInfo.HIV.gov. (n.d.). Rechallenge.
Giles, A., et al. (2019). Sulfonamide allergies. Pharmacy.
Ovung, A., et al. (2021). Sulfonamide drugs: structure, antibacterial property, toxicity, and biophysical interactions. Biophysical Reviews.
Vally, H., et al. (2012). Adverse reactions to the sulphite additives. Gastroenterology and Hepatology From Bed to Bench.
Wulf, N. R., et al. (2013). Sulfonamide cross-reactivity: Is there evidence to support broad cross-allergenicity? American Journal of Health-System Pharmacy.
