Decongestants and cough syrup that contain Codeine like Robitussin are not considered liquid codeine because they contain other active ingredients. Codeine syrup or cough syrup containing Codeine is made to fight the symptoms of flu and cold. Guaifenesin is an active ingredient in codeine syrup which softens mucus so that it can be coughed up. Pheniramine is an antihistamine which reduces the symptoms of colds like runny nose and sneezing.