If you have alcohol in your system, it can affect your judgment and your muscle control. The effects of alcohol sedate you and it can make it difficult for you to control yourself while driving. In the U.S., if your blood alcohol concentration or BAC is over .08% then you are more likely to be the victim of a crash.
AAA explains what affects your BAC.
Strength and size of the drink
If you drink slowly over some time, your BAC may be lower than someone who drinks all of their alcohol within a short amount of time. The strength and size of your drink will also contribute to your BAC. If you drink alcohol with a high alcohol percentage, then you will feel the effects of drunkenness more than someone with a lower percentage. Likewise, a larger drink may have more alcohol than a smaller drink. The best way to keep from having a high BAC is to pace yourself and to drink smaller, less alcohol heavy drinks.
Gender and body weight
Alcohol breaks down differently in the bloodstream for men and women. One reason for this is that women produce a smaller amount of the enzyme, dehydrogenase, which breaks down alcohol than men do. If men and women drink the same amount of alcohol over the same amount of time, the woman may have a 20% higher BAC.
Heavier people also tend to have a lower BAC for alcohol that they drink compared to a smaller person. The reason that heavier people may not have a high BAC is that the alcohol mixes with the body fluids and people with higher body weight have more fluids.