Antihistamines that cross the blood brain barrier make people feel stoned or tired because histamine does something entirely different in the brain from what it

Antihistamines that cross the blood brain barrier make people feel stoned or tired because histamine does something entirely different in the brain from what it does in the rest of the body. Seems like there's a lot of code reuse in the brain. It operates almost like a different sort of biology encapsulated within an animal body.