Please tell me do you have acid reflux disease?
Since how long you have mettalic taste and is it started along with cough?
Do you have mercury filled teeth?
Last question please metallic taste started with cough is it?and it was not prior to cough...
Thanks for info as you have no history of acid reflux disease ,mercury filled teeth ,mettalic taste just started after cough and you have cough for last three weeks keeping your history in mind it has been seen that sometime metallic taste in mouth with cough can be because of respiratory tract infection ,as you are on cough medication along with allergy pills but still problem is there i will suggest you once to consult Pulmonologist (chest dr) to examine your respiratory tract and to rule out the possibility of infection that can be the possible reason of your problem.