As a clinical pharmacist, it's important to understand sulfonamide "sulfa" allergy. Many medications containing sulfonamide chemical group have labels that warn against using them in patients who have had an allergic reaction to sulfonamides in the past. There is a potential for cross-reactivity between members of a specific class (e.g, two antibiotic sulfonamides). However, recent findings suggest that the risk of cross-reactivity between antibiotic and non-antibiotic sulfonamides is very low or may not occur at all.
Allergic reactions involving antibody production (anaphylaxis) are unlikely with non-antibiotic sulfonamides. On the other hand, T-cell-mediated reactions (type IV) such as maculopapular rash are not well understood, and it's currently not possible to completely rule out this potential. In cases where previous reactions were severe, such as Stevens-Johnson syndrome or toxic epidermal necrolysis (TEN), some clinicians may choose to avoid using medications from these classes.
This is still listed as a contraindication in the labeling for celecoxib (Celebrex), but studies don't support it. Now interaction software is starting to follow that evidence. There's lots of confusion about which drugs are actually sulfonamides and how likely they are to cross-react.
- Sulfonylarylamines. These are the sulfa antibiotics (TMP/SMX, sulfadiazine, sulfacetamide), sulfasalazine and the HIV protease inhibitors, darunavir (Prezista) and fosamprenavir (Lexiva). These are the most likely to cause a sulfa allergy. And cross-sensitivity is possible even with topical and ophthalmic products.
- Nonsulfonylarylamines. These drugs have a structure that is similar to the sulfonylarylamines, but different enough that cross-sensitivity is rare. They include thiazide and most loop diuretics, sulfonylureas and celecoxib.
- Sulfonamide moieties. These drugs have a sulfonamide group, but their structure is quite different. This group includes sumatriptan, topiramate, zonisamide, and the new hepatitis C protease inhibitor, simeprevir (Olysio).
Ask about the drug and reaction if patients report a sulfa allergy. If a patient had a mild rash or hives with TMP/SMX, advise avoiding other sulfonylarylamine antibiotics. But explain that it's usually okay to take a nonsulfonylarylamine or a sulfonamide moiety. Advise avoiding all classes of sulfonamides if the reaction was life-threatening (anaphylaxis, Stevens-Johnson syndrome, etc). Tell patients not to worry about drugs or foods that contain sulfur, sulfate, or sulfites. These are NOT sulfonamides, so they're okay in patients with sulfa allergies. Dapsone is also not a sulfonamide, it's a sulfone. Although it has a low risk of cross-reactivity with sulfonamides, avoid it in patient who have severe sulfa allergies.
REFERENCES
Wulf, N.R. and Matuszewski, K.A. (2013). Sulfonamide cross-reactivity: Is there evidence to support broad cross-allergenicity? American Journal of Health-System Pharmacy, 70(17), pp.1483–1494. Available at: https://academic.oup.com/ajhp/article-abstract/70/17/1483/5112341?redirectedFrom=fulltext&login=false
Shapiro, L.E., Knowles, S.R., Weber, E., Neuman, M.G. and Shear, N.H. (2003). Safety of Celecoxib in Individuals Allergic to Sulfonamide. Drug Safety, [online] 26(3), pp.187–195. Available at: https://link.springer.com/article/10.2165%2F00002018-200326030-00004.