Phenylephrine is a selective α1-adrenergic receptor agonist used primarily as a decongestant, as an agent to dilate the pupil, and to increase blood pressure. Phenylephrine is marketed as a substitute for the decongestant pseudoephedrine, though clinical studies differ regarding its effectiveness in this role. Phenylephrine is used as a replacement for pseudoephedrine in decongestant medicines due to pseudoephedrine's use in the illicit manufacture of methamphetamine. Its efficacy as an oral decongestant has been questioned, with multiple studies not being able to come to an agreement. Whereas pseudoephedrine causes both vasoconstriction and increase of mucociliary clearance through its nonspecific adrenergic activity, phenylephrine's selective α-adrenergic agonism causes vasoconstriction alone, creating a difference in their methods of action.
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