phenylephrine UK /ˌfiː.nʌɪlˈɛf.ɹɪn/US /ˌfɛn.əlˈɛf.ɹiːn/ noun1 •An α-adrenergic receptor agonist related to adrenaline, used as a vasoconstrictor and nasal decongestant; 3-(1-hydroxy-2-methylamino-ethyl)phenol, with the formula C₉H₁₃NO₂. formsphenylephrines