 [ Symptoms ] is breaking out in a rash a side effect of amoxicillin or an allergic reaction my 8 month old broke out in a rash after 3 days of using amoxicillin for an ear infection doctor said it was a side effect i just want to make sure cause my sister has a high allergy to amoxicillin [ Disease ] an allergic reaction is often sudden hives that are very pruritic itchy an amoxicillin side effect usually occurs near the end of the course of treatment day 8 10 not day three and is bright red occuring all over the body this does not usually itch so it could be either one i do not have enough specific information and the ability to examine your baby to offer you a definitive answer