text: Alcohol exerts a lot of effects in the brain.  The reason that it dehydrates you is that it affects hormones in the brain.  We all have a hormone called antiduirectic hormone in our bodies.  It is released in times of dehydration to help you to hold on to water.  One of alcohol's effects is to stop the release of antiduirectic hormone.  Therefore, alcohol stops you from releasing a hormone to help you hold on to water.  The bottom line, you pee more than you drink.