From JAMA Patient Page, edited and condensed:
Most people who report allergies to penicillin (PCN) are not allergic. About 1 in 10 patients has a PCN allergy noted in their medical record. Many such allergies are diagnosed in childhood, often because of a rash that may have been caused by a virus, not an allergy. Even among patients with true PCN allergy, 8 in 10 are no longer allergic within a 10-year period. It is important to find out if you are really allergic to PCN. Having an unverified PCN allergy may result in other antibiotics being used, which may increase the risk of treatment failure or more dangerous side effects.
If you have had a reaction to PCN such as hives, rash, swelling, or shortness of breath – especially if it has been many years since the reaction – and you wish to learn if you are still truly PCN allergic, please ask to be referred to an allergist for evaluation.