The thinking on this is divided. Roger Boulton, a fermentation and distillation chemistry expert and professor in the University of California–Davis’ Viticulture & Enology Department, says they don’t. “The current thinking is that dehydration is responsible for hangover,” he says. Drinking equal amounts of identical-proof vodka and Cognac will get you just as drunk, and will make you equally hungover the next morning, he says.
However there is also the belief, voiced in a 2000 study of hangover research in the Annals of Internal Medicine, that brown liquors do cause worse hangovers. The reason? Alcohols in the booze other than ethanol, commonly referred to as congeners.
A distiller tries to isolate the “good” alcohol, or ethanol, from the “impurities,” including congeners, contained in the soupy fermented material he’s distilling (corn, barley, or rye for whiskey; sugarcane for rum; etc.). These include methanol, or wood alcohol, which can make you go blind if you drink too much of it, and a group of compounds called fusel oils. The distiller will do this by throwing out the first and last parts of what comes out of the still (known as the heads and tails), and in some cases by redistilling the alcohol to strip it further.
Though all liquor, regardless of whether it’s destined to be Cognac, whiskey, or vodka, comes out of the still clear (barrel aging turns it brown but doesn’t add any congeners), distillers making brown liquors usually don’t strip their booze of as many of the impurities as those making a clear spirit. While vodka in particular is supposed to be flavorless and odorless, brown spirits don’t have to live up to such a standard of purity. Congeners, although they might cause worse hangovers, all produce flavor. And when it comes to brown spirits, that’s a good thing.