What is the mechanism of action of Diphenhydramine (Benadryl)?

Diphenhydramine, commonly known by the brand name Benadryl, primarily acts as an H1 antagonist. This means it blocks the histamine H1 receptors in the body. Histamine is a compound involved in various physiological processes, especially in allergic responses. By inhibiting the action of histamine at these receptors, diphenhydramine effectively reduces symptoms of allergies, such as itching, runny nose, and swelling.

Additionally, since H1 receptors are also found in the central nervous system, diphenhydramine can produce sedative effects due to its ability to cross the blood-brain barrier and interfere with histamine's role in maintaining wakefulness. This property makes it useful not only for allergic reactions but also as a short-term sleep aid.

The other options present different mechanisms that are not applicable to diphenhydramine. For instance, a 5HT-3 antagonist would relate to serotonin pathways, while a D2 antagonist would target dopamine receptors, both of which are not the primary action of diphenhydramine. Opioid receptor agonists interact with opioid receptors in the nervous system, which is also unrelated to the antihistaminic properties exhibited by diphenhydramine. Understanding the specific