I just called American Airlines 2-3 days ago with the same question.  We are taking a trip to Cancun 3/30 and wanted to be sure I could carry it on with me.  They said I needed to have a note from the doctor stating that I would be carrying the medicine and syringes with me.  Be sure to carry the same number of syringes/needles as you have vials of medicine.  Hope this helps!