Is guaifenesin a decongestant?

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.

No.: Guaifenesin is thought to act as an expectorant by increasing the volume and reducing the viscosity of secretions in the trachea and bronchi.