Can you take a maximum strength nasal decontgestant with Robitussin dm?

Robitussin DM (guaifenesin and dextromethorphan) Contains a cough suppressant (dextromethorphan) and an expectorant (guaifenesin). Adding a low dose decongestant should not cause any interactions.
Shouldn't. You are already taking a decongestant as the "d" of "dm" so more is just likely to cause side effects. Try vaporizer or other means of opening you up.