A hangover is the sum of a set of biochemical effects that are triggered inside the body when alcohol is consumed. All these effects take place in different physiological areas of the body and, hence, a hangover is not just restricted to the head. That headache is a manifestation of collective biological reactions to alcohol. Let’s take an area-by-area tour of the geography of a typical hangover.

Vasopressin Inhibition → Diuretic Effect → Dehydration
An anti-diuretic hormone, vasopressin is responsible for sending chemical signals to the kidneys to not send all the water to the bladders for expulsion and retain some of it for the body to reabsorb. This process is extremely important for saving the body from dehydration. Vasopressin plays an important role in regulating the levels of water, glucose and salts in the bloodstream. Now, when alcohol hits the bloodstream, its molecules are carried all over the body including the pituitary gland (where the preprohormone or precursor of vasopressin is produced) and the hypothalamus (where this preprohormone is synthesized). Alcohol inhibits the production of the precursor of vasopressin in the pituitary gland and this leads to a diuretic effect as the kidneys no longer receive chemical signals for water reabsorption. All the water from liquid materials that enter the body get carried over to the bladders wherefrom they are evacuated. This lowers the level of water in the body, making way for dehydration. Remember the number of trips you took to the restroom the last time you emptied two bottles of beer? Well, the reason was nothing but the suppression of vasopressin which caused more water to fill your bladder than usual. The resultant dehydration is what causes those nagging headaches, signaling that your body needs to replenish the excessive loss of fluids. This is the reason why it is recommended that you drink lots of water before and in-between drinks.

Alcohol Toxins → Inability to Metabolize → Nausea and Vomiting
During the fermentation process of wines and liquors, certain chemical compounds, known as congeners, are produced which are believed to impart distinct aromas to distilled liquors and signature tastes to non-distilled alcoholic beverages. Common alcoholic congeners are aldehydes, acetaldehyde, acetone, fusel alcohols, ester alcohols, etc. Certain toxic congeners, such as acetaldehyde, are produced in the body itself when the liver metabolizes alcohol with the help of an enzyme, alcohol dehydrogenase. Acetaldehyde is further broken down by two other enzymes, acetaldehyde dehydrogenase and glutathione, to form harmless compounds. However, during binge-drinking sessions, the rate at which alcohol and toxic congeners are broken down into nontoxic compounds is slower than the rate of alcohol intake. Glutathione, especially, takes a lot of time to get replenished and in case of heavy drinking, a lot of alcohol and acetaldehyde get to stay in the body in their toxic forms for a long time before getting broken down. Owing to the presence of such toxins in the body and the liver’s inability to metabolize them at a decent pace, it is common for hangover symptoms such as nausea and vomiting to accompany headache. This is nothing but the body’s attempt to expel the excess alcohol and toxins.