Hangover

A hangover is the response to poisoning by small quantities of complex wood alcohols that are generally found in forms of alcohol that are aged for long periods of time in wood, such as rye, scotch and wine. Although grain alcohol can be metabolized by the liver at a fairly quick rate (about 1/2 fl.oz. of pure alcohol per hour, about the amount in one beer), the more complex alcohols are essentially toxic, take longer to break down, and are highly toxic to the body. As such, they tend to be present long after the original alcohol has been metabolized. The by products of the metabolism of the complex alcohols cause headaches and the nausea typical of any type of poisoning. In addition, alcohol tends to dehydrate the body in any event, leading to dry mouth and "fuzzy tongue".
There is no treatment for a hangover, but the symptoms can be addressed. Fluids are advised to deal with the dehydration, but coffee and more alcohol should be avoided: juice and water are better. For the headache, acetaminophen is recommended as aspirin tends to exacerbate the nausea. However, the best treatment is rest until the symptoms go away on their own.
In The Mistake, Robert Chase told Sam McGinley that he had been suffering from a hangover when he treated Sam's sister Kayla McGinley to goad him into filing a lawsuit against the hospital but in actuality, Chase was suffering from grief caused by the surprise phone call that had told him that his father had died.
Hangover at Wikipedia