Hair of the dog
"Hair of the dog" is a colloquial expression in the English language predominantly used to refer to alcohol that is consumed with the aim of lessening the effects of a hangover. The expression originally referred to a method of treatment of a rabid dog bite by placing hair from the dog in the bite wound. The use of the phrase as a metaphor for a hangover treatment dates back to the time of William Shakespeare. Ebenezer Cobham Brewer writes in the Dictionary of Phrase and Fable (1898): "In Scotland it is a popular belief that a few hairs of the dog that bit you applied to the wound will prevent evil consequences. Applied to drinks, it means, if overnight you have indulged too freely, take a glass of the same wine within 24 hours to soothe the nerves. 'If this dog do you bite, soon as out of your bed, take a hair of the tail the next day.'" He also cites two apocryphal poems containing the phrase, one of which is attributed to Aristophanes. It is possible that the phrase was used to justify an existing practice, and the idea of Latin: similia similibus curantur ("like cures like") dates back at least to the time of Hippocrates. In the 1930s cocktails known as Corpse Revivers were served in hotels.
The phrase also exists in Hungarian, where the literal translation to English is "(You may cure) the dog's bite with its fur", but has evolved into a short two-word phrase ("kutyaharapást szőrével") that is used frequently in other contexts when one is trying to express that the solution to a problem is more of the problem. Among the Irish and Mexicans, the phrase 'The Cure' ("curarse la cruda", in Spanish) is often used instead of 'hair of the dog'. It is used, often sarcastically, in the question "Going for a Cure?" In Costa Rica (Central America) the same expression is used but it refers to a pig as in: hair of the same pig ("pelos de la misma chancha" in Central America) referring to the same method to cure the hangover.
In some Slavic languages (Polish, Bosnian, Croatian, Serbian) hair of the dog is called "a wedge" (klin), mirroring the concept of dislodging a stuck wedge with another one; hence the popular Polish phrase "[to dislodge] a wedge [with] a wedge" - [wybijać] klin klinem - which is used both with regard to alcohol and figuratively. The proper Russian term is – опохмел (opohmelka, "after being drunk"), which indicates a process of drinking to decrease effects of drinking the night before.
In German, drinking alcohol the next morning to relieve the symptoms is sometimes described as "having a counter-beer" (ein Konterbier trinken), whereas in Austria people talk about having a repair-beer (Reparatur-Seidl).
A similar usage is encountered in Romanian, in the phrase "Cui pe cui se scoate"; in Bulgarian, in the phrase "Клин клин избива"; in Italian, in the phrase "Chiodo scaccia chiodo"; and in Turkish, in the phrase "Çivi çiviyi söker". In all four cases the English translation is "a nail dislodges a nail", though these phrases are not exclusively used to refer to the hangover cure.
In Swedish, drinking alcohol to relieve a hangover is called having an "återställare", which translates roughly to "restorer". In Norwegian, it is usually called "å reparere", meaning "to repair/fix". In Finnish, it is called "tasoittava" (leveling) or "korjaussarja" (repair kit) and in Czech "vyprošťovák" (extricator). In Tanzania, the equivalent Swahili phrase used is "kuzimua" which means 'assist to wake up after a coma'.
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 [ethanol] 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 coincides with the rate of 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.[clarification needed]
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.
- Hair of the dog on MedTerms
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|Look up hair of the dog in Wiktionary, the free dictionary.|
- "Early Modern Whale: Langley Marish: Milton, and the hair of the dog" a compilation of early uses of the expression "hair of the dog".