Hangover - Causes of a hangover
NHS Choices Medical Reference
Many of the symptoms associated with a hangover are caused by dehydration. Alcohol makes you dehydrated because it is a diuretic (makes you produce more urine).
Alcohol causes salts to be removed from the blood, which draws water out of the blood and into the kidneys, where it is passed out through the urine.
Alcohol can also cause malnutrition because it uses up the body's store of vitamins and minerals and reduces blood sugar levels. Together with dehydration, this can be responsible for the throbbing headache you may experience with a hangover.
Alcohol can also cause blood vessels in the head to dilate (become wider), adding to the pain of a hangover headache.