Alcohol harms the DNA.
In our body, alcohol is first converted to acetaldehyde, and then to acetic acid, which, unlike acetaldehyde, is completely harmless and which serves as a source of energy in further metabolic processes. And acetaldehyde makes us suffer from a hangover - it, as the researchers from Cambridge found, also damages DNA. Experiments with blood stem cells showed that acetaldehyde tears both strands of DNA; as they say in the article in Nature, if there are a lot of such gaps, the cells do not have time to patch them, and as a result, uncorrected mutations appear in the genome, which the dividing cells transmit to their descendants.
Mutations often lead to cancer. And, perhaps, it is because of this peculiarity in the gene that people in South-East Asia are so often sick with esophageal cancer: their enzyme does not work well, therefore, too much acetaldehyde accumulates in the cells, which creates oncogenic mutations.