[Image Source] FLASHBACK Why does benadryl make you sleepy? Most allergy sufferers know the debate about whether it would be better to deal with allergy symptoms (itchiness, runny nose, watery eyes, etc.) or to take a benadryl and be sleepy for the rest of the day.  Why does choosing have to be so difficult? Benadryl is a competitive antagonist of histamine.  Histamine is a small molecule that when it attaches to the histamine receptor causes a variety of symptoms.  Levels of histamines rise drastically when you come in contact with an allergen (my biggest weakness is dust).  What does it mean to be a competitive antagonist?  Antagonist means it stops histamine from being able to work on its receptor.  Competitive means it binds to the receptor instead, preventing histamine from binding and exerting its action. H1 Receptors, the histamine receptors that benadryl can bind to and block histamine from, cause allergy symptoms, but they also promote arousal (meaning wakefulness here).  So when you block them, you lose that wakefulness and become sleepy.

[Image Source] FLASHBACK

Why does benadryl make you sleepy?

Most allergy sufferers know the debate about whether it would be better to deal with allergy symptoms (itchiness, runny nose, watery eyes, etc.) or to take a benadryl and be sleepy for the rest of the day.  Why does choosing have to be so difficult?

Benadryl is a competitive antagonist of histamine.  Histamine is a small molecule that when it attaches to the histamine receptor causes a variety of symptoms.  Levels of histamines rise drastically when you come in contact with an allergen (my biggest weakness is dust).  What does it mean to be a competitive antagonist?  Antagonist means it stops histamine from being able to work on its receptor.  Competitive means it binds to the receptor instead, preventing histamine from binding and exerting its action.

H1 Receptors, the histamine receptors that benadryl can bind to and block histamine from, cause allergy symptoms, but they also promote arousal (meaning wakefulness here).  So when you block them, you lose that wakefulness and become sleepy.