Phenylephrine acts as a direct alpha-1 adrenergic agonist that causes blood vessel constriction, which provides effective nasal vasoconstriction when properly administered.
The claim specifies that phenylephrine acts as a nasal decongestant by constricting blood vessels, which relates to its pharmacological mechanism as an alpha-1 adrenergic agonist. Papers [3] and [6] directly support its ability to induce vasoconstriction (in nasal mucosa and other vascular beds) via alpha-adrenergic mechanisms. Although oral phenylephrine's clinical efficacy as a decongestant has been debated due to first-pass metabolism (as noted in systematic reviews like [10]), its fundamental mechanism of action as a vasoconstrictor is well established in pharmacological literature.