Sulfa Allergy: Separating Fact from Fiction

About 2-3 % of people have had an allergic reaction to a “sulfa” antibiotic, most typically sulfamethoxazole/trimethoprim, also known as Bactrim or Septra. Pharmacists and physicians used to recommend staying away from all sulfa-containing medications if you’d had allergic reactions to Bactrim (TMP/SMX) –but not anymore.  Let’s sort out myth from facts for folks with a “sulfa allergy.”

A “sulfa allergy” is a term applied for adverse reactions and symptoms caused by the antibiotic sulfonamide, TMP/SMX, most often prescribed for urinary tract infections. While there are other sulfonamide antibiotics, they aren’t typically prescribed in the U.S.  

How do I know if I have a sulfa allergy? 

If you were prescribed Bactrim (trimethoprim/sulfamethoxazole) for an infection and had an allergic reaction including fever or rash 7 to 14 days after starting the antibiotic, you have a sulfa allergy. 

What is a “sulfa drug?” 


The term sulfa drug has contributed to some of the ongoing confusion about whether a reaction to the antibiotic TMP/SMX means will react to all sulfa drugs. It turns out the answer is no; you don’t need to avoid all sulfa drugs.

What about sulfa drugs that aren’t the antibiotic TMP/SMX?

There are subtle differences in non-antibiotic sulfonamides such that they do NOT cause an immune-mediated reaction. Sulfonamides that are not antibiotics do not show evidence of cross-reactivity— meaning they should not cause an allergic reaction in people with an allergy to TMP/SMX.  

If I have a reaction from Bactrim, can I take……

1.. Acetazolamide.  Prescribed for the prevention of altitude sickness, yes, you can. With any new medication, watch for rash or severe side effects.
2. Celecoxib (Celebrex). An anti-inflammatory commonly used for musculoskeletal pain, yes, you can take that. 
3. The diuretics Bumetanide (Bumex) and Furosemide (Lasix).  Doctors prescribe them for lower extremity swelling and heart failure. Yep, you can take these
4. Diabetes medications Glimepiride, Glipizide and Glyburide. Yep.
5. Thiazide diuretics Chlorthalidone and HCTZ treat high blood pressure and are OK to take even if you have an allergy to Bactrim. 
6. Migraine medications, the “triptans” Eletriptan, Frovatriptan, Sumatriptan, etc., are safe in those with reported sulfa allergy. 
7. Antibiotic Eyedrops: Gentamicin sulfate, Polytrim (Polymyxin B sulfate/trimethoprim), Atropine sulfate, Neomycin (Maxitrol), and other sulfur-based eye drops are safe to use in those with self-reported sulfa allergy. There is one eye drop you should NOT use and that is Blephamide eye drops.

To sum it up: the evidence from clinical trials suggests that folks who have had an allergic reaction to the generic antibiotic TMP/SMX do not need to avoid all sulfa-type drugs, including those listed above. 


Hope this helps

Dr. O.