Scientific Background
There are at least two theories as to how "hair of the dog" works:
In the first, hangovers are described as the first stage of alcohol withdrawal, which is then alleviated by further alcohol intake. Although “...Low doses may effectively prevent alcohol withdrawal syndrome in surgical patients”, this idea is questionable as the signs and symptoms of hangover and alcohol withdrawal are very different.
In the second, hangovers are attributed to methanol metabolism. Levels of methanol, present as a congener in alcohol, have been correlated with severity of hangover and methanol metabolism to the highly toxic formate via formaldehyde has a timecourse in keeping with the appearance of hangover symptoms. As both ethanol and methanol are metabolised by alcohol dehydrogenase - and ethanol is a much better substrate for this enzyme - drinking more of the former then effectively prevents (or delays) the metabolism of the latter. As pure ethanol consumption has also been found to increase endogenous levels of methanol presumably for this reason, this suggests that if “hair of the dog” works in this way it effects a temporary hiatus rather than a cure.
From the perspective of sugar metabolism, alcohol may cause a blood sugar spike, resulting in a hypoglycemic awakening "hangover". Finishing the drink left on the counter might be the quickest way to ingest more calories that quickly convert to sugar to raise the body's blood sugar, as well as lifting the fainting and headaches often associated with low blood sugar.
One common feature of hangovers (and excessive alcohol consumption in general) is nausea caused by the irritating corrosive chemical properties of ethanol on the human gastrointestinal mucosa, rather than a buildup of bloodborne toxins; this effect is actually increased by further alcohol consumption, and would likely result in increased damage (and more nausea/vomiting) if more alcohol is ingested before the mucosa can regenerate itself. In these cases, "hair of the dog" would actually be highly harmful and likely make things worse.
Read more about this topic: Hair Of The Dog
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