Robitussin is an antitussive as Robitussin contains two active ingredients which are dextromethorphan and guaifenesin.
The Dextromethorphan in Robitussin is an antitussive drug that is used to help relieve constant coughing.
It helps stop your coughing by decreasing activity in your brain that triggers your impulse to cough.
Guaifenesin is an expectorant and not an antitussive.
An antitussive is a cough suppressant which helps keep you from coughing when you're not needing to or wanting to cough.
The expectorant which is what Guaifenesin is helps you cough up mucus and other things when you need to clear your lungs.
Guaifenesin is an expectorant that helps to thin and loosen the mucus in your lungs thus making it easier to cough up the mucus.
The Antitussives are cough suppressants that affect a certain part of the brain (cough center), reducing the urge to cough.
Decongestants relieve stuffy nose symptoms.