Guaifenesin and dextromethorphan are the active ingredients in Robitussin DM cough syrup, according to MedicineNet. Each 5 milliliters of the syrup contains 100 milligrams of guaifenesin and 10 milligrams of dextromethorphan.
Guaifenesin is an expectorant that thins oral mucus while increasing its volume, explains MedicineNet. This helps loosen chest and throat congestion, making it easier to expel through coughing, notes Drugs.com.
Dextromethorphan is a cough suppressant, advises Drugs.com. It acts on cough receptors in the brain and interferes with the cough reflex, states MedicineNet.
A combination medicine, Robitussin DM treats congestion and coughs due to colds and allergies, according to Drugs.com. It is ineffective for coughs related to smoking, emphysema and asthma.