Going back many years to DUI enforcement training...

It seems to me we were taught that impurities in the alcoholic beverages, some of which were natural and some of which were added, were the main cause of the headaches. Chemical changes in the substance that occurred during the fermentation process also were a cause.

Those additives and impurities are also why people develop that distinct odor on their breath after drinking most alcoholic drinks. More difficult to detect that odor with more pure spirits like highly distilled vodka.

But that was over 20 years ago. My memory may not be correct.