There are many possible explanations for how alcohol causes cancer. It is likely that it causes different types of cancer in different ways.
Here are the most likely theories:
Acetaldehyde, the hangover chemical
In your body, alcohol is converted into a toxic chemical called acetaldehyde. This is one the main reasons why we get hangovers.
But acetaldehyde does more than give you a headache in the morning. It can cause cancer by damaging DNA and stopping our cells from repairing this damage.
Acetaldehyde also causes liver cells to grow faster than normal. These regenerating cells are more likely to pick up changes in their genes that could lead to cancer.
Oestrogen and hormones
Alcohol can increase the levels of some hormones, such as oestrogen, testosterone and insulin. Unusually high levels of oestrogen increase the risk of breast cancer.
Drinking lots of alcohol can damage the cells of the liver, causing a disease called cirrhosis. Cirrhosis makes you more vulnerable to liver cancer.
Alcohol makes it easier for the tissues of the mouth and throat to absorb the cancer-causing chemicals in tobacco. This is one reason why people who smoke and drink multiply the damage they receive and have especially high risks of cancer.
Folate (wiki - Vitamin B9) is an important vitamin that helps our cells produce new DNA correctly. People who drink alcohol tend to have lower levels of folate in their blood and some studies have found that cancer is more common in people with low folate levels.