Allergy to penicillin and related antibiotics is the most commonly reported drug allergy in the United States. An allergy to penicillin is the only one that can be diagnosed through skin testing. Allergy testing can take anywhere from 2 to 3 hours including the time that is needed after the testing to watch for reactions. Your testing will include 2 types of skin tests as well as an oral challenge with penicillin, if deemed necessary.