Sulfonamides (Sulfa Drugs): Uses, Full Drug List, Side Effects and Allergy Facts
Updated: 6 days ago
Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026 · How we review
Quick Answer: What are sulfonamides?
Sulfonamides, also called sulfa drugs, are synthetic medicines that share a sulfur-containing part in their chemical structure. Sulfonamide antibiotics stop bacteria from making folate, and other sulfonamides treat high blood pressure, diabetes, gout, migraine, seizures and HIV. About 3% to 8% of people are allergic to a sulfonamide, but one such allergy doesn't mean you're allergic to all. [1]
What the research says
A sulfonamide antibiotic allergy rarely means a true cross-allergy to other sulfonamides. In a study of 969 people who reacted to a sulfonamide antibiotic, 9.9% later reacted to a non-antibiotic sulfonamide, compared with 1.6% of 19,257 people without that history. Their risk of reacting to penicillin was even higher (adjusted odds ratio 3.9 vs. 2.8 for non-antibiotic sulfonamides), which points to a general tendency toward allergies rather than true cross-reactivity between these drugs. (Strom et al., N Engl J Med, 2003) [2]
Many people labeled allergic to sulfonamide antibiotics can safely take the drug again. In a prospective study of hospitalized adults with a low-risk sulfonamide antibiotic allergy history, a supervised single-dose test (direct oral challenge) with trimethoprim-sulfamethoxazole was safe, and 75 of 76 people (99%) had the allergy label removed. (Mitri et al., Infect Control Hosp Epidemiol, 2026) [3]
Key takeaways
Sulfonamides are synthetic drugs named for a sulfur-containing part of their structure.
They block folate production in bacteria; humans get folate from food, so we are not affected the same way.
They treat dozens of conditions, from infections to diabetes, blood pressure and HIV.
Bactrim contains a sulfonamide. Celebrex (celecoxib) is a sulfonamide.
Severe skin reactions are rare but serious. Report any blistering rash or mouth sores right away.
What are sulfonamides?
Sulfonamides are a class of synthetic medicines, meaning they are made in a lab and do not occur naturally. [1]
All sulfonamides share one feature: a specific part of their chemical structure that contains sulfur. That shared part gives the group its name, and it is why people call them "sulfa drugs."
Despite that shared piece, these drugs can be very different from one another. Some fight bacteria. Others lower blood pressure, control blood sugar or prevent seizures.
Sulfonamide vs. sulfite vs. sulfur: An allergy to sulfonamides is not the same as a reaction to sulfites in food or wine, or to the element sulfur.
How do sulfonamides work?
Sulfonamide antibiotics work by blocking how bacteria make folate, also called vitamin B9, which bacteria need to grow and multiply. [1] [4]
Here is the process, step by step:
Bacteria build their own folate inside their cells.
The sulfonamide blocks a key step in that process.
Without folate, bacteria essentially starve and stop multiplying.
Sulfonamides do not actually kill bacteria directly. They stop them from growing.
Why don't they harm us the same way? People mainly get folate from food rather than making it. So sulfonamides usually do not affect human cells the way they affect bacteria, except in certain rare medical conditions. [1]
What is the history of sulfa drugs?
Sulfonamides became widely available as antibiotics in the mid-1930s and were among the best antibiotics for about a decade, until penicillin overtook them in the mid-1940s. They were not the very first antibiotics, but they changed modern medicine. [1]
After they became widely available, deaths from infectious diseases in the U.S. fell by 28%.
Sulfonamides never disappeared. Instead, chemists adapted their core structure into many new medicines for completely different conditions.
Period | What happened |
Mid-1930s | Sulfonamides become widely available as antibiotics |
~1935–1945 | Among the best antibiotics available; infection deaths in the U.S. drop 28% |
Mid-1940s | Penicillin use expands and overshadows sulfonamide antibiotics |
Today | More than 40 sulfonamide drugs approved in the U.S. for dozens of conditions |
[1]
What do sulfonamides treat?
Sulfonamides treat bacterial infections, such as urinary tract infections, and a huge range of other conditions, from type 2 diabetes and high blood pressure to HIV and melanoma. At first, they were used only to treat and prevent bacterial infections. [1]
Which sulfonamides are antibiotics?
These FDA-approved sulfonamide antibiotics are used to fight bacteria:
Sulfonamide antibiotic | Common brand names |
Mafenide | Sulfamylon® |
Silver sulfadiazine | Silvadene®, Thermazene® |
Sulfamerazine | — |
Sulfameter | — |
Sulfamethizole | — |
Sulfanilamide | — |
Sulfaphenazole | — |
Sulfapyridine | — |
Sulfathiazole | — |
Sulfisoxazole | — |
[1]
Which sulfonamides are not antibiotics?
These FDA-approved sulfonamides are grouped below by what they treat. Most of them are not antibiotics.
Condition or use | Sulfonamide drugs (brand names) |
High blood pressure | Chlorthalidone; hydrochlorothiazide; indapamide; torsemide (Soaanz®) |
Diuretic (water pill) | Bendroflumethiazide (Corzide®); chlorothiazide (Diuril®); hydroflumethiazide; methyclothiazide; polythiazide; trichlormethiazide; ethoxzolamide |
Heart failure and fluid buildup (edema) | Bumetanide (Bumex®); furosemide (Furoscix®, Lasix®); indapamide; metolazone; torsemide |
Kidney and liver disease | Furosemide; metolazone; torsemide |
Type 2 diabetes | Acetohexamide; chlorpropamide; tolazamide; tolbutamide |
Chronic low blood sugar | Diazoxide |
High cholesterol | Rosuvastatin (Crestor®) |
Pulmonary arterial hypertension | Bosentan (Tracleer®); sildenafil (Revatio®) |
Erectile dysfunction | Sildenafil (Viagra®) |
Arthritis and period pain | Celecoxib (Celebrex®) — osteoarthritis, rheumatoid arthritis, primary dysmenorrhea |
Gout and high uric acid | Probenecid |
Ethoxzolamide | |
Migraine | Sumatriptan (Imitrex®, Onzetra®, Tosymra®) |
Seizures (focal epilepsy) | Zonisamide (Zonegran®, Zonisade®) |
Low-potassium paralysis | Dichlorphenamide (Keveyis®, Ormalvi®) |
Ulcerative colitis | Sulfasalazine (Azulfidine®) |
Acne and pink eye | Sulfacetamide (Klaron® lotion; eye drops) |
Malaria | Sulfadoxine (treatment; less often prevention) |
HIV | Darunavir (Prezista®) |
Metastatic melanoma | Vemurafenib (Zelboraf®) |
Enlarged prostate (BPH) | Tamsulosin (Flomax®) |
[1]
Is my medicine a sulfa drug? (Bactrim, Celebrex and others)
Celebrex, Lasix, hydrochlorothiazide, Flomax and Imitrex are all sulfonamides, while Bactrim contains one sulfonamide alongside a non-sulfonamide partner. These are the medicines people ask about most often:
Medicine | Is it a sulfonamide? | What to know |
Bactrim™ | Partly | Contains the sulfonamide sulfamethoxazole plus trimethoprim, which works in a similar way but is not a sulfonamide |
Celebrex® (celecoxib) | Yes | A non-antibiotic sulfonamide used for arthritis pain |
Lasix® (furosemide) | Yes | A non-antibiotic sulfonamide diuretic |
Hydrochlorothiazide | Yes | A non-antibiotic sulfonamide for blood pressure |
Flomax® (tamsulosin) | Yes | A non-antibiotic sulfonamide for enlarged prostate |
Imitrex® (sumatriptan) | Yes | A non-antibiotic sulfonamide for migraine |
[1]
If you are unsure whether your own medicine is a sulfonamide medication, your pharmacist can check the ingredients in seconds.
Can you be allergic to sulfonamides?
Yes, about 3% to 8% of people worldwide are allergic to one of the sulfonamides, a rate similar to penicillin allergy. [1]
The good news: researchers now know that an allergy to one sulfonamide does not automatically mean an allergy to all of them. [2]
If you have had an allergic reaction to a sulfonamide:
Tell your doctor and pharmacist which drug caused it and what happened.
Ask whether an alternative sulfonamide is safe for you.
Ask whether a medicine from a different drug class would work instead.
For more on drug allergy labels and testing, see our Penicillin Allergy: The Complete Guide.
What are the side effects of sulfa drugs?
The main side effects and drawbacks of sulfonamides are rare but severe skin reactions, crystals in urine, and bacteria that are naturally immune or become resistant to these antibiotics. [1]
Drawback | What it means for you |
Many bacteria are naturally immune | The drug may not work for some infections |
Bacteria can become resistant | Overuse makes these antibiotics less effective over time |
Rare but severe skin reactions | Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) can be life-threatening |
Crystals in urine | Uncommon and often preventable |
[1]
These adverse reactions and risks vary widely from one medicine to another. Your healthcare provider is the best source of advice for your specific medicine.
Antibiotic resistance is a growing global problem. Learn how it plays out with other bacteria in our guide to carbapenem-resistant Enterobacterales (CRE).
Can sulfonamides cause crystals in urine?
Yes, sulfonamides can sometimes cause crystals to form in urine, but this is not a common side effect, and you may be able to avoid it. [1]
Ask your provider whether your specific drug is likely to cause crystals and what you can do to lower the risk. You can also read the official prescribing label for any U.S.-approved medicine on DailyMed, the U.S. National Library of Medicine's drug label database.
When should you call your doctor about a sulfa drug reaction?
Call your doctor or seek urgent care if you take a sulfonamide and notice a blistering or spreading rash, sores in your mouth, eyes or genitals, fever with a new rash, or swelling of the face or trouble breathing. Watch for:
A rash that blisters, peels or spreads quickly
Sores in your mouth, eyes or genitals
Fever together with a new rash
Swelling of the face or trouble breathing
These can be early signs of a severe reaction, such as Stevens-Johnson syndrome. Stop the medicine only as your provider directs, and get help right away.
Conclusion
Sulfonamides have more than 90 years of history behind them. They started as some of the first antibiotics and have grown into a family of medicines that treat dozens of conditions, from head to toe.
Most people take sulfa drugs safely. If you have had an allergic reaction before, you are not automatically barred from every sulfonamide. The key is to tell your care team exactly what happened, so they can choose the safest option for you.
Have questions about a medicine you take? Bring this guide to your next appointment and ask your doctor or pharmacist to review your list. For more clear, research-backed health guides, explore more guides on Rinnit.com and subscribe for new articles every day.
FAQ
What is a sulfonamide in simple terms?
A sulfonamide is a lab-made medicine with a sulfur-containing part in its structure. Some are antibiotics, while others treat conditions like high blood pressure, diabetes and migraine.
How do sulfonamides kill bacteria?
They don't kill bacteria directly. They block the bacteria from making folate (vitamin B9), so the bacteria starve and cannot multiply.
Is Bactrim a sulfa drug?
Partly. Bactrim contains the sulfonamide sulfamethoxazole plus trimethoprim, which works in a similar way but is not a sulfonamide.
Is Celebrex a sulfa drug?
Yes. Celecoxib (Celebrex) is a non-antibiotic sulfonamide used for arthritis pain and period pain.
If I'm allergic to Bactrim, can I take furosemide or hydrochlorothiazide?
Possibly. An allergy to one sulfonamide does not mean you are allergic to all of them. Your doctor or pharmacist can assess your history and suggest a safe option.
How common is sulfa allergy?
About 3% to 8% of people worldwide are allergic to a sulfonamide, a rate similar to penicillin allergy.
What are the most serious side effects of sulfa drugs?
Rare severe skin reactions, such as Stevens-Johnson syndrome and toxic epidermal necrolysis, are the most serious. Seek urgent care for a blistering rash or mouth sores.
Are sulfa drugs still used today?
Yes. More than 40 sulfonamide drugs are still approved in the U.S. for dozens of conditions.
What are the symptoms of a sulfa allergy?
The most common signs are rash, hives and itching. Rarely, a severe reaction causes fever, blistering or peeling skin, or swelling and trouble breathing; call 911 for trouble breathing or swelling of the face or throat.
Is a sulfa allergy the same as a sulfite or sulfur allergy?
No. Sulfa drugs, sulfites in foods and wine, and the element sulfur are different substances. A sulfa drug allergy does not mean you will react to sulfites or sulfur.
Related reading on Rinnit
References
Cleveland Clinic. What Are Sulfonamides (Sulfa Drugs)? Uses, Types, Side Effects & Examples. Cleveland Clinic. Last updated February 26, 2025. https://my.clevelandclinic.org/health/treatments/sulfonamides
Strom BL, Schinnar R, Apter AJ, et al. Absence of cross-reactivity between sulfonamide antibiotics and sulfonamide nonantibiotics. N Engl J Med. 2003;349(17):1628-1635. PMID 14573734. https://doi.org/10.1056/NEJMoa022963
Mitri EA, Vogrin S, Hall R, et al. Inpatient direct oral challenge for sulfa antibiotic allergy: improving care in immunocompromised hosts. Infect Control Hosp Epidemiol. 2026;47(4):409-413. PMID 41623154. https://doi.org/10.1017/ice.2026.10400
Ovung A, Bhattacharyya J. Sulfonamide drugs: structure, antibacterial property, toxicity, and biophysical interactions. Biophys Rev. 2021;13(2):259-272. PMID 33936318. https://doi.org/10.1007/s12551-021-00795-9
Medical disclaimer
This article is for general education only and is not a substitute for professional medical advice, diagnosis or treatment. Always talk with your doctor or pharmacist before starting, stopping or changing any medicine.
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