K.L.
My niece got hives from using an antibiotic, and they were BAD! She took the medication for 10 days before the hives showed up, and they ended up lasting for over a month. She did the benadryl as well as got some shots at the doctor because at one point her lips swelled up and they thought it was spreading to her throat.
Her doctor also recommended using aveno baby products. It did give her some relief.