Is guaifenesin a decongestant?

No. Guaifenesin is thought to act as an expectorant by increasing the volume and reducing the viscosity of secretions in the trachea and bronchi.
Not necessarily. No, not per se. 'decongestant' usually refers to medications like sudafed, afrin, etc. That open up your airways. Mucinex (guaifenesin) works by loosening up the mucous so you can hack/snort/cough it out. It is safe to use if you have high blood pressure, whereas the others can raise your blood pressure.

Related Questions

Can I take sudafed (pseudo ephedrine only), Mucinex (guaifenesin) (expectorant and decongestant), and nyquil all at once?

Probably ok. Sudafed and Mucinex (guaifenesin) certainly can be taken together and adding nyquil at bedtime is probably not harmful. Your pharmacist who knows all the ingredients and actions of these medications is very qualified to advise you,however. Read more...