Chemicals produced in the body from alcohol include ethanol, methanol, formaldehyde and formic acid. Each of these can have a detrimental biological effect on the body and are discussed in more detail.
Ethanol is converted into acetaldehyde by the enzyme ADH, and then ALDH catalyses the breakdown of acetaldehyde into acetic acid. If the first reaction occurs at a faster rate than the second, then acetaldehyde accumulates. The suggestion that acetaldehyde accumulation is involved in hangovers is largely due to the observation that high concentrations of acetaldehyde in the blood give rise to toxic effects which resemble hangovers (rapid pulse, sweating, nausea, etc.).
One chemical that has been identified as potentially important in hangover is methanol. Methanol is absorbed and metabolised by the same mechanisms as ethanol, but ethanol is preferentially metabolised when both substances are present. Methanol levels in the blood therefore remain high after ethanol levels decrease, possibly explaining the delayed onset of hangover.
The products of methanol breakdown are formaldehyde and formic acid, both of which are very toxic: high concentrations can cause blindness and death. Drinks which are associated with increased hangover symptoms contain high levels of methanol (e.g. brandy, whisky).