While penicillin is a highly effective antibiotic, like all medications, it can cause side effects. Most side effects are mild and manageable, but some can be serious. Common adverse reactions include gastrointestinal issues like diarrhea, nausea, and vomiting, as well as skin rashes and itching. These often resolve on their own once the medication course is completed.

A significant concern with penicillin is the potential for allergic reactions. It's estimated that a large percentage of people who believe they are allergic to penicillin are not, a phenomenon known as 'penicillin allergy' being incorrectly documented. True penicillin allergies, mediated by IgE antibodies, can range from mild skin rashes and hives to severe, life-threatening anaphylaxis, characterized by breathing difficulties, swelling, and a drop in blood pressure. It is crucial for individuals to consult with a healthcare provider or allergist for proper diagnosis, often involving skin testing, to confirm an allergy.

Identifying a true allergy is vital because incorrectly labeling someone as allergic to penicillin can lead to the unnecessary use of broader-spectrum antibiotics, which may be less effective, have more side effects, and contribute to antibiotic resistance. For those who are confirmed to be allergic, alternative antibiotics are readily available. Healthcare providers play a critical role in assessing reported allergies and 'de-labeling' patients when appropriate, ensuring that penicillin can be safely used when indicated, thereby preserving its efficacy and availability for those who can benefit from it.