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"Hair of the dog" is a colloquial English expression 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.
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 Spanish) referring to the same method to cure the hangover.
In Polish, hair of the dog is called "a wedgie" (klin), mirroring the concept of dislodging a stuck wedge with another one. Similarly, other Slavic languages, such as Russian, Bosnian, Croatian, Bulgarian, Serbian and Slovene, use the phrase "a wedge dislodges a wedge" (клин клин избива in Bulgarian, klin se klinom izbija and клин клином выбивают), although it is not normally connected to the alleged hangover medicine. The proper Russian term is - Опохмел ("after being drunk"), which indicates a process of drinking to decrease effects of drinking the night before.
A similar usage is encountered in Romanian, in the phrase "Cui pe cui se scoate", and in Italian, in the phrase "Chiodo scaccia chiodo". In both 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".
The phrase "hair of the dog" was apparently first employed for a hangover cure in KTU 1.1114 line 29 wherein the chief god of the Ugaritic pantheon, 'i/el, takes some for his health. The usage is a borrowing from Akkadian.
This phrase is used by Jack Nicholson in the movie The Shining by director Stanley Kubrick when chatting with the bartender during the ghost party at the hotel. In the French version, Nicholson says: "De quoi reprendre du poil de la bête". The Dutch version says: "'n Borrel tegen de haarpijn". The Italian version is totally different and has no relation with the English phrase.
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 formaldehydehas 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.
- Hair of the dog on MedTerms
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- "Early Modern Whale: Langley Marish: Milton, and the hair of the dog" a compilation of early uses of the expression "hair of the dog".