How common or rare is an allergic reaction or serious complication from amoxicillin?

5 to 10 percent of people are allergic to Amoxicillin (which is a type of penicillin) People with allergies like asthma, urticaria and hay fever, are highly prone to develop medication allergies. Reactions can include rashes, hives, and anaphylaxis. If you are a highly allergic person, make sure you have benadryl or an epipen handy for any tongue swelling or difficulty breathing.