Benadryl and cough medicine...which should I give her?

My baby is 1 1/2....Dr. says she has a cold and to just use the cough medicine (DM) she perscribed months back. She has a miserable cough from the mucus and is very hoarse so I was thinking of giving her benadryl to dry her up. It always worked before but I have been scared to try it because she has been taking this cough syrup...how many hours should I wait before admin the benadryl...she had the cough medicine last at 4:30 and her nose is still pouring.