Robitussin Dm (Guaifenesin / Dextromethorphan) is a combination medicine. Guaifenesin is an expectorant that breaks up and thins out mucus. Dextromethorphan is a cough suppressant. Robitussin (Guaifenesin) is an expectorant. It contains guaifenesin, which thins and loosens mucus, making it easier to cough out.