Alcohol causes dehydration because it inhibits a hormone called anti-diuretic hormone (ADH.) This hormone has a constant level in the body and keeps you from peeing out all of the water in your body. When you drink alcohol, the level of this hormone becomes low and you pee more. While this happens, you are also losing salt. One alcoholic drink can result in up to a pint of urine production. This effect becomes less as you drink more. But, if you drink 15 drinks in a night, you can end up well over two liters dehydrated.