Vasopressin regulates the body's water retention and constricts blood vessels. Without it, the kidneys send water straight to the bladder and you straight to the line for the bar bathroom.
It's not pee you're worried about the next morning. According to gastroenterologist Dr. Kathlynn Caguiat, "Alcohol can increase gut motility and it doesn't get broken down before it reaches the colon, where bacteria feast on these, resulting in bloating and diarrhea." Those bacteria love the alcohol you feed them, and they repay you with extra gas and crap.
What's more, increased gut motility means everything's moving more rapidly than usual. That in turn means the colon has less time to absorb any water, which can result in watery stool and diarrhea.
And the worst offenders are…
"The more concentrated the alcohol, the worse the reaction will be," Shah warns, which explains the severe reactions whiskey can provoke the next morning. Liquors will produce the worst booze craps, in other words.