Pseudoephedrine (PEP)

Pseudoephedrine is a sympathomimetic amine. Its principal mechanism of action relies on its indirect action on the adrenergic receptor system. The vasoconstriction that pseudoephedrine produces is believed to be principally an α-adrenergic receptor response. While it may have weak or no 'direct' agonist activity at α- and β-adrenergic receptors, the principal mechanism is to cause the release of endogenous norepinephrine (noradrenaline) from storage vesicles in presynaptic neurons. The displaced noradrenaline is released into the neuronal synapse, where it is free to activate the postsynaptic adrenergic receptors. These adrenergic receptors are located on the muscles lining the walls of blood vessels. When these receptors are activated by noradrenaline, the muscles contract, causing the blood vessels to constrict (vasoconstriction).