Allergic reactions to penicillin antibiotics are common. Cephalosporins are other antibiotics that fight many of the same infections as penicillin. They may be an option for patients who are allergic to penicillin. However, cephalosporins and penicillins have similar chemical structures. Some patients who are allergic to penicillin are also allergic to cephalosporin, but information about the frequency of cephalosporin allergy in patients with penicillin allergy is limited. With some drugs, such as penicillin and cephalosporin, doctors can perform a “skin test” for allergic reactions by administering small amounts of the drug under the skin. However, rather than perform a skin test, many doctors simply avoid cephalosporin in patients with penicillin allergy.