Phenylephrine hydrochloride, or NeoSynephrine, is a synthetic sympathomimetic drug compound structurally similar to epinephrine that is a selective agonist of the α1-adrenergic receptor on blood vessels, causing vasoconstriciton. Its in-vivo effects include decongestion, hypertension, and pupil dilation. It is a common ingredient in many decongestants, and for cold, flu, allergy, or breathing illnesses.