Getting drunk usually refers to drinking alcohol. Most of the alcohol we drink is in the form of ethanol, as a part of beer, wine or spirits.
Ethanol is water soluble, which means it gets absorbed through the small intestine and into the blood stream. In the blood stream it reaches the brain, which is where it causes the effects that make a person feel drunk.
The actual process of it affecting the brain are quite complicated and not fully understood, but the effects on the body are experienced by lots of people every night! General effects are euphoria, poor balance, loss of coordination, reduced inhibition, and erratic behaviour. But increased consumption can cause vomiting and in severe cases result in a coma or death!
Gradually the ethanol is removed from the body by excretion, or by being metabolised in the same way as food. It is the excretion of a small amount of alcohol that allows breathalyser testing to work, and the amount of alcohol in the blood that sets the drink-drive limit in the UK.
Alcohol from the drinks (or sometimes food!) gets into your bloodstream through your stomach and intestine, which means it’s very quickly transported around your body and reached your brain fast! Most of the alcohol is absorbed into your blood stream in your intestine, and happens faster if you’ve not eaten recently.
Alcohol (or specifically, as John says, ethanol) stays in tour body the liver has broken it down. It can only leave your body through breathing, sweating and your urine. That’s why you can often smell if someone has been drinking.
So why does it change people’s behaviour?
The frontal lobe in your brain is the first part that is affected by the ethanol in your blood stream, and this bit of your brain is responsible for judgement and rational reasoning.
So if that bit of the brain is being affected, it explains why people often do things they wouldn’t usually d or start to lose their judgement of distance and become unstable on their feet.