I have an uncle who is the other extreme of the spectrum, he drinks nothing but beer, but has done so for so many years that is doesn't affect him at all. He was also a professional truck driver and his driving skill shows it. We can be raking the hay field and his rows after a case of beer will be straighter and nicer than mine without me ever dribking a drop.
But the majority of people are social drinkers who may have a low blood alcohol level, even a legal one yet their reaction times may be slower since they are not used to it.