Mechanism of action
Diphenhydramine functions as an inverse agonist at H1 receptors, and subsequently reverses effects of histamine on capillaries, reducing allergic reaction symptoms .The H1 receptor is similar to muscarinic receptors. Therefore, diphenhydramine also acts as an antimuscarinic; it is a competitive antagonist of muscarinic acetylcholine receptor, resulting in its use as an antiparkinson medication