When Should Mucolite Be Given For A Cold And Cough In A Toddler, And Could It Cause A Runny Nose And More Coughing??

Hello doctor,at what stage of cold / cough should mucolite be given? My 18 month old baby did not have a runny nose or much phlegm when I started mucolite. But after 4 dosrs of 2 ml each morning and night ,she is coughing and has very runny nose. I am confused if and when I can use mucolite drops..please help.