 Congeners include substances such as amines, amides, acetones, acetaldehydes, polyphenols, methanol, histamines, fusel oil, esters, furfural, and tannins, many but not all of which are toxic.One study in mice indicates that fusel oil may have a mitigating effect on hangover symptoms, while some whiskey congeners such as butanol protect the stomach against gastric mucosal damage in the rat.Different types of alcoholic beverages contain different amounts of congeners.