(or the acetate ion) can cause additional problems. One study found that injecting sodium acetate into rats caused them to have nociceptive behavior (headaches). In addition, there is a biochemical explanation for this finding. High acetate levels cause adenosine to accumulate in many parts of the brain. But when the rats were given caffeine, which blocks the action of adenosine, they no longer experienced headaches. Congeners In addition to ethanol and water, most alcoholic drinks also contain congeners, either as flavoring or as a by-product of fermentation and the wine aging process. While ethanol is by itself sufficient to produce most hangover effects, congeners may potentially aggravate hangover and other residual effects to some extent. 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. In general, dark liquors have a higher concentration while clear liquors have a lower concentration. Whereas vodka has virtually no more congeners than pure ethanol, bourbon has a total congener content 37 times higher than that found in vodka. Several studies have examined whether certain types of alcohol cause worse hangovers. All four studies concluded that darker liquors, which have higher congeners, produced worse hangovers. One even showed that hangovers were worse and more frequent with darker liquors. In a 2006 study, an average of 14 standard drinks (330 ml each) of beer was needed to produce a hangover, but only 7 to 8 drinks was required for wine or liquor (note that one standard drink has the same amount of alcohol regardless of type). Another study ranked several drinks by their ability to cause a hangover as follows (from low to high): distilled ethanol diluted with fruit juice, beer, vodka, gin, white wine, whisky, rum, red wine and brandy. One potent congener is methanol. It is naturally formed in small quantities during fermentation and it can be accidentally concentrated by improper distillation techniques. Metabolism of methanol produces some extremely toxic compounds, such as formaldehyde and formic acid, which may play a role in the severity of hangover. Ethanol slows the conversion of methanol into its