What is risk of reaction if allergic to pcn and taking cephalosporin antibiotic?

Some risk, but: The risk of an allergic reaction to cephalosporins in a penicillin-allergic patient is generally reported at between 7-18%. However, cephalosporins are very often given to patients who report an allergy to penicillin without any reactions. The exception is the patient with an anaphylactic reaction. You can be tested for confirmation of true penicillin and cephalosporin allergy.

Cephalosporin rx: Older studies suggested that there was a 5-7% risk of having a reaction to a cephalosporin if you had a history of penicillin allergy. However, there have been recent studies indicating that the newer cephalosporins are structurally different enough that the risk of cross reaction is quite low, even eliminating the need to test for penicillin allergy prior to administering the cephalosporin.