DEAR MAYO CLINIC: As a kid, I remember having a rash after taking penicillin. To this day, my doctor prescribes something other than penicillin if needed when I am ill. I have heard that people who think they are allergic to penicillin may not be. Should I check whether I truly am allergic? ANSWER: Penicillin refers to two things. One is a specific antibiotic prescribed to treat some common illnesses. Penicillin also refers to a…