In penicillin allergy, the immune system mistakenly reacts to penicillin as if it were a harmful substance and releases compounds like histamines into the bloodstream, according to the American Academy of Allergy, Asthma & Immunology (AAAAI).
These compounds cause hives (a raised, itchy rash) and swelling around the face, hands and feet. Doctors typically treat penicillin allergy with antihistamines and sometimes corticosteroids. Rarely, people can have a life-threatening reaction to penicillin called anaphylaxis, which requires immediate treatment with epinephrine, the hormone in EpiPens. Further treatments may include albuterol to relax and open the airways, IV fluids and corticosteroids.
An allergic reaction to penicillin typically occurs less than an hour after someone receives a dose of the antibiotic, according to the AAAAI.
Around 10% of the U.S. population report having a penicillin allergy, but rough estimates suggest that less than 1% of the population may be truly allergic to this class of antibiotic, according to the CDC. And the CDC notes that 80% of people with a valid diagnosis lose their sensitivity to penicillin within 10 years.
Doctors can confirm a penicillin allergy using a skin prick test, during which a small amount of the antibiotic is injected into the skin. If an itchy bump appears within 30 minutes of the test, the patient is likely allergic to penicillin. Individuals who test positive may be prescribed a different antibiotic medication, according to Yale Medicine.
However, if penicillin is absolutely necessary — for example, when a life-threatening infection has no therapeutic alternatives — a patient may need drug desensitization treatment. This involves administering progressively greater doses of penicillin every 15 to 20 minutes until a full therapeutic dose is reached, allowing the immune system to temporarily tolerate the drug.