Which leads to another theory that puts the blame for the hangover on pure physiology. Alcohol is first broken down in the liver into a toxic substance called acetaldehyde. Acetaldehyde is then further broken down into a harmless substance called acetate. At high doses, acetaldehyde causes nausea, vomiting, sweating and other symptoms akin to the hangover. Although there is no acetaldehyde in your system when you have a zero blood-alcohol level, some of the after-effects of the toxin may persist the morning after.