…that makes you pee an inhuman amount and shrinks your brain… When alcohol enters your bloodstream, it tells your pituitary gland not to produce vasopressin, which is the hormone that typically keeps your body lubed up with moisture. Without vasopressin, liquids get siphoned straight to your bladder, which is why you really open the floodgates after the first time you pee during a boozy evening. When you're drinking, you lose about four times more liquid than you gain, which also causes the dehydration that leads to that wonderful cotton mouth and headache that come with a hangover. Ever wonder why exactly dehydration causes a headache? It's because your organs are so desperate they steal water from your brain, which causes your brain to shrink. A shrunken brain pulls on the membranes that connect the brain to the skull, and that, naturally, hurts like a mother.
You also feel weak and generally depleted because all that peeing rids your body of salt, potassium, and magnesium which are important for normal nerve, muscle, and cell function. Alcohol also breaks down and makes you pee out glycogen, which in a non-drunk is a natural source of energy in your liver.
… and produces toxic chemicals in your body that make you feel terrible… When we metabolize alcohol, our livers create an extremely toxic substance called acetaldehyde. Humans have handy enzymes that break down acetaldehyde called acetaldehyde dehydrogenase and glutathione. But when we drink too much, the enzymes can't keep up, and the toxin builds up, increasing the severity of headaches and nausea. To give you an idea of how toxic acetaldehyde is: Antabuse, a drug used in severe cases of alcoholism, completely blocks the acetaldehyde-eating enzymes. The result is that even a drop of alcohol causes a flushed face, headache, nausea, vomiting, chest pain, weakness, blurred vision, mental confusion, sweating, choking, breathing difficulty, and anxiety.
Glutamine, on the other hand, is suppressed while we're knocking back champers. Glutamine is a natural stimulant, so when we stop drinking, our body gets busy trying to replenish glutamine levels, which is why we usually sleep restlessly after a night of debauchery. It's called glutamine rebound, and in severe cases it causes a special hangover featuring tremors, anxiety, and high blood pressure.
… sometimes depending on what you drink… Red wine and dark liquors like whiskey, brandy, bourbon and tequila produce more impurities, called congeners, during fermentation than lighter colored alcohol like wine, rum and vodka. Here's a handy congener chart. Congeners basically make all hangover symptoms worse. You maybe have noticed that although it's clear, bubbly gives a nasty headache. That's because carbonation increases your blood's absorption of alcohol.
...and in what order… Because carbonation increases your blood alcohol absorption, that saying "beer before liquor, never sicker; liquor before beer, you're in the clear" is at least partly right. Beer's carbonation can set up a mainlining effect if you switch to hard liquor. The "in the clear" part, however, is mostly wishful thinking.
Also, that drinking buddy who's always warning you: Bro, don't mix? He is correct. Mixing various types of alcohol on a single night is unwise if your functioning potential the following day is important. Each alcohol produces different types of congeners which combine to create a strong cocktail of morning-after pain.
… but you can minimize the pain if you're diligent. There's no single magical cure. Hair of the dog only delays the inevitable by making you drunk and oblivious again. Burnt toast is a myth. But several strategies can at least minimize your hangover misery:
Eat: a full stomach before drinking absorbs alcohol and protects your stomach lining. Breakfast the next day including eggs, bananas and fruit juice contains important hangover combatants like cysteine, potassium and fructose. A sports drink can help replace electrolytes, sugars and salt lost with all the peeing. But as much as you crave coffee, caffeine will only make you more dehydrated.
Hydrate: drink one glass of water per alcoholic drink.
Take multivitamins: they'll combat the vitamin depletion that comes with urinating every five minutes.
Don't mix: See above.
Take two aspirin: with water before bed and in the morning to reduce inflammation.
UPDATE: Unless you absolutely can't help it, don't puke. Keith Roussil of the Rocky Mountain Poison and Drug Center says making yourself throw up won't do much good because alcohol is absorbed in the bloodstream so quickly. And getting chunk caught in your lungs could cause aspiration pneumonia, which could kill you.
Happy New Year everyone!