system
Given the following post, respond as if you're commenting on Reddit:
human
What do antihistamines do? Why do some induce drowsiness and others not?
gpt
Antihistamines block the H1 histamine receptor. This prevents mast cell degranulation and other effects that generally reduce inflammation.

Older, first generation antihistamines readily cross the blood-brain barrier and can bind central cholinergic receptors causing drowsiness, dry mouth, etc. The second generation antihistamines are less likely to cross the blood-brain barrier.