About Phenylephrine An a1-adrenergic receptor agonist, Phenethylamine derivative, Decongestant, Antiglaucoma, Mydriatic. Mechanism of Action of Phenylephrine The drug exerts it`s pharmacological action by acting as a selective alpha-1 adrenergic agonist. It causes vasoconstriction and increases total peripheral resistance and raises blood pressure. It reduces intraocular tension by constricting ciliary body blood vessels and produce mydriasis (without cycloplegia). It exerts nasal decongestant action- by vasoconstrictory action on arterioles of nasal mucosa. It prolongs and localizes the action of anaesthetics due to it`s vasoconstrictory action on skin, mucous membrane, and viscera and thus slows the rate of absorption of local anaesthetics.