The effects of alcohol use can be different between men and women because of differences in body chemistry. Women usually have less body water than men, so the alcohol a woman drinks is less diluted in her body; the effect is greater. Women also tend to have less of the enzyme that breaks down alcohol in the stomach before the alcohol has a chance to enter the bloodstream.
Also, how a woman's body breaks down alcohol is influenced by changes in hormones over her menstrual cycle. A woman drinking alcohol at different times in her menstrual cycle will have different BAC readings, even though the amount of alcohol consumed is the same. The use of oral contraceptives can also influence how a woman's body breaks down alcohol.