I have allergic reaction from antibiotic cephalexin. what is the best antihistamine and how long will it take to clear up, also now have swollen face?

Have you been seen?: This is an urgent situation. Hopefully, you have been seen by a physician. If not, proceed to the ER if the swelling involves the mouth (potential for progression to throat and breathing incapacity). If you are improved, then an urgent visit in the morning (or Virtual Consult) is recommended for prednisone (for phenomenon of delayed reaction). Cetirizine 10mg twice a day if no contraindications