Is There A True Allergy?

Take a good history! The best method to determine if a patient is allergic to penicillin is with history

Allergic reactions require a minimum of 7 days before IgE antibodies to an antigen can form. Therefore, patients cannot have an allergic reaction on the first course of an antibiotic until 7 days has passed, as no IgE antibodies have been formed

IgE-mediated reactions typically occur within 1 hour of IV administration or up to 2 hours after oral administration, though in exceedingly rare cases, this reaction can be more delayed

Isolated GI upset or maculopapular rashes are rarely IgE-mediated reactions; rather, they are far more likely to be a side effect of antimicrobial use in general, or secondary to viral infection, respectively