Phenylephrine is a nasal decongestant that provides relief from nasal discomfort caused by colds, allergies, and hay fever.It works by reducing swelling of the blood vessels in the nose.Phenylephrine relieves symptoms but doesn’t treat the cause of the symptoms or speed recovery.It’s the active ingredient in dozens of generic products, including children’s cough medications and over-the-counter (OTC) drugs.henylephrine is a nasal decongestant that provides relief from nasal discomfort caused by colds, allergies, and hay fever.