Allergic reactions to drugs are common and may involve shock (anaphylaxis) or rashes (urticaria) as well as swelling (angioedema).  It is critical for patients with an ongoing drug allergy to be evaluated by an allergist as the responsible drug must be identified and discontinued immediately.  Patients allergic to some drugs can be tested to confirm the presence of an allergy and alternative medications for future use may be selected.  In the rare case that an alternative drug is not an option, allergists can desensitize patients to the required drug.