How can you tell if your baby is allergic to antibiotics? is it possible to get an allergic reaction after 7 days of using it?

Usually rash: For most people a rash will be the clue to an antibiotic allergy. However, some rare patients can have a more severe reaction called anaphylaxis, which can be life threatening. The whole concept of "allergic" reaction requires exposure to a potential allergen and for the body's immune system to develop a sensitivity. This can happen after one dose or after many doses.

History & exam fit: Allergic reaction to meds usually involves typical itchy rash (measles like or hives). From first exposure to onset of reaction usually 7-10 days and on second exposure within minutes to hours. Get baby to experienced allergist who can examine baby and get detailed history. Except for penicillin there is no reliable skin testing for meds so history, exam are key. Take clear close-up pics of rash.