It is surprising that, given how widespread hangovers are (especially on New Year's Day), there is not more scientific consensus on what causes the phenomenon.
Smithsonian.com, however, points to the most recent studies on the subject, including the "most compelling theory," which is that hangovers result
from a buildup of acetaldehyde, a toxic compound, in the body. As the body processes alcohol, acetaldehyde is the very first byproduct, and it’s estimated to be between 10 and 30 times as toxic as alcohol itself. In controlled studies, it’s been found to cause symptoms such as sweating, skin flushing, nausea and vomiting.
Read more here.
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