Penicillin and related antibiotics, including amoxicillin, are among the most frequently reported drug allergies. Reactions vary from a mild itchy rash that appears during or after a course, to hives and swelling, through to rare but serious anaphylaxis with breathing difficulty and a drop in blood pressure. Some reactions appear within an hour of a dose, while others develop over days.
It is worth knowing that many people labelled as penicillin-allergic turn out not to be truly allergic when properly assessed, for example because a childhood rash was due to the infection rather than the drug. Carrying an inaccurate penicillin-allergy label can lead to less effective or more costly antibiotics being used, so it can be valuable to have a suspected allergy formally reviewed.
If you think you have reacted to an antibiotic, stop taking it and contact a doctor or pharmacist for advice, and always tell healthcare staff and pharmacists about the suspected allergy before starting any new medicine. Seek emergency care immediately for swelling of the face or throat, breathing difficulty, widespread rash with blistering, or feeling faint. A doctor or allergy clinic can assess whether the allergy is genuine and advise on safe alternatives.