Although a glass or two of red wine a day are thought to be beneficial, excessive alcohol consumption - perhaps defined as that which produces a hangover - can increase the risk of cancer.
The body produces acetaldehyde to detoxify the ethanol in alcohol, but acetaldehyde is itself toxic. It can damage certain stretches of DNA, causing mutations leading to cancer. The body also produces an enzyme to protect itself against acetaldehyde, but production capacity is limited. It is when production of acetaldehyde exceeds production of the enzyme that the risk of cancer increases.

Excessive alcohol consumption is linked particularly to cancers of the liver, larynx and oesophagus.