In the US you can buy alcohol in flight, but its against the law to drink your own personal alcohol if you brought any onto the plane

If you fly first class, you typically can get a complimentary alcoholic beverage while everyone else is boarding, and then also get another once the plane is in flight.

Thats why on occasion if theyre offering first class upgrade for like $80-$100 Ill do it, especially as a tall person. The extra space is great, and if youre already planning to drink in an airport bar during a layover then its basically moot. Airport bars charge like $20 for a watered down cocktail with less alcohol in it than the drinks an attendant will make

My wife always gets an old fashioned on plane rides in the US.