Partie 1:
As it turns out, it's a combo of both.
Blame it on the Ethanol
When it comes to the severity of your headache, your principal nemesis is ethanol—a.k.a. ethyl alcohol, says Michael Oshinsky, Ph.D., of Thomas Jefferson University. Ethanol is the colorless, flammable liquid that puts the buzz in your favorite beer, wine, or liquor. And it doesn’t really matter what type of ethanol you knock back, Oshinsky explains. It’s all about how much of it you consume, and how quickly you consume it.
The ethanol in your drink ends up mostly in your liver, where it’s metabolized into a compound called acetaldehyde—which is then converted into a chemical called acetate. This conversion happens all over your body, including in your brain tissues, Oshinsky’s research shows. And those circulating levels of acetaldehyde and acetate—and the inflammation they cause—are the root of your headache
The more booze you down in a short period of time, the more of those chemicals your brain has to contend with, and the more your noggin throbs. So it’s not the tequila or those aggressively-hopped IPAs that cause your headache. It’s the fact that you take your Patron straight and all at once—and your ales by the pint—that explains the vice clamped around your temples the next day. (Some alcohol-heavy beers pack 9 or 10 percent, which is closer to wine than light beer. So imagine slamming a pint of pinot in 15 minutes.)
Ethanol is also a diuretic—meaning it makes you pee more. More urination leads to dehydration and electrolyte imbalances, which also contribute to your sore head, says Janne Tolstrup, Ph.D., of the University of Southern Denmark.
(Meanwhile, see The Side Effect of Drinking That's Even Worse Than a Hangover.)
The Second Culprit: Congeners
You can also blame your upset stomach, shakiness, thirst, and generally foul feeling on compounds called congeners—which are basically toxins left behind by the fermenting and aging process, explains Damaris J. Rohsenow, Ph.D., of Brown University. And not all drinks are created equal when it comes to congener content.
Most of your alcohol’s color—whether light tan or dark brown—comes from the barrels they’re aged in. And those barrels are also the source of most congeners, Rohsenow explains. That means the darker your hooch, the more toxins have likely seeped into it. Bourbon—aged in charred oak barrels—tops the list when it comes to liquor congeners, Rohsenow explains.
Close on bourbon’s heels are Scotch and the various types of whiskey—American, Canadian—which are also barrel-aged. At the bottom of the list are gin and vodka—the latter of which is often filtered multiple times to remove contaminants. (Rohsenow didn’t look at tequila. But the brown reposado or añejo varieties—as opposed to silver or blanco tequilas—are aged in oak barrels, and so likely contain congeners similar to whiskey.) Flavor additives—the “limon” or “peppar” in your rum or vodka—may also contain congeners, and so could exacerbate your hangover, she says.
Most beers contain many more congeners than liquor. For example, Budweiser packs 142 milligrams (mg) of congeners per 12-ounce can compared to just 49 mgs in one scotch drink, Rohsenow says. Different types of alcohol contain different congeners, and whiskey’s may be more hangover-inducing than beer’s, even though they’re less numerous. But not many guys cite Bud as a big hangover inducer, so congeners aren’t your head’s worst enemy. 
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