The law says 0.08% Blood Alcohol Content, in most states in the US. How much alcohol that takes depends on a lot of factors such as body weight and time period. For example, if you weigh 100 lb that is 2 drinks If you weigh 200 lb that is 4 drinks Reference has some charts. .
Just one is all it takes to impair your mind set. Well that's what they say anyway. Who's they you wonder? Those who have a title like Dr. or those who wear a police uniform.
People's response to alcohol varies widely. THere are some people, I think 'tissue tolerant' is the term, who do not get drunk as they drink, up to the point they die of alcohol poisoning.
