Overview of trimethoprim+sulfamethoxazole hypersensitivity in adults

The most common hypersensitivity reaction to sulfonamides is a delayed skin reaction characterised by erythema, maculopapular or morbilliform rash, urticaria and pruritus. In most cases the immunological mechanism involved is unknown, but it is not thought to be IgE-mediated.

Allergy to most nonantibiotic sulfonamides (eg furosemide) does not imply allergy to antibiotic sulfonamides (eg sulfamethoxazole). In patients who are allergic to a nonantibiotic sulfonamide (with the exception of sulfasalazine), an antibiotic sulfonamide can be used (see Cross-reactivity between sulfonamides for more detail).