A gene that helps drink go to your head has been discovered by scientists.

As well as providing a cheap night out, it is believed to protect against alcoholism.

Previous research has shown that people who react strongly to alcohol are less likely to become alcoholics.

The gene, CYP2E1, provides the coded instructions for making an enzyme that breaks down alcohol.

Scientists found that 10% to 20% of the population possess a particular version of the gene that causes them to get drunk easily.

The first few drinks during a night out will leave these individuals feeling more inebriated than their friends.

Drugs that enhance the effect of CYP2E1 could in future be used to sensitise people to alcohol before an evening's drinking - or even sober them up when they have had one too many, said the researchers.

Scientists in the US investigated the genetics of 237 college student siblings who had one alcohol-dependent parent but were not alcoholics themselves.

They homed in on an end region of chromosome 10 where the CYP2E1 gene resides.

Participants' response to drinking was linked to their genetic make-up. Students were given a mixture of grain alcohol and soda that was equivalent to about three average alcoholic drinks. At regular intervals they were then asked whether they felt drunk, sober, sleepy or awake.

Senior study author Professor Kirk Wilhelmsen, from the University of North Carolina at Chapel Hill, said: "We have found a gene that protects against alcoholism, and on top of that, has a very strong effect.

"But alcoholism is a very complex disease, and there are lots of complicated reasons why people drink. This may be just one of the reasons."

There are three distinct variations of the chromosomal region containing CYP2E1, said Prof Wilhelmsen.

He added: "We can make predictions about the level of response to alcohol an individual will have based on determining which haplotypes an individual has. Figuring out which specific variations are important would be icing on the cake. It is likely that a combination of variations is what is important."

Most of the alcohol in the body is metabolised by another enzyme, alcohol dehydrogenase, which works in the liver.

CYP2E1's effect on sobriety is probably due to the fact that it is not active in the liver, but the brain.

It also generates destructive molecules called free radicals, which can damage sensitive structures such as brain cells.

"It turns out that a specific version or allele of CYP2E1 makes people more sensitive to alcohol, and we are now exploring whether it is because it generates more of these free radicals," said Prof Wilhelmsen.

"This finding is interesting because it hints at a totally new mechanism of how we perceive alcohol when we drink. The conventional model basically says that alcohol affects how neurotransmitters, the molecules that communicate between neurons, do their job. But our findings suggest it is even more complex than that."

The findings are published today in an early online edition of the journal Alcoholism: Clinical and Experimental Research.

In another study published in the same journal, scientists found evidence that the brain's ability to become addicted to alcohol depends on genetic make-up.

The research, conducted on mice, showed changes associated with addiction in animals lacking a key "feel good" gene.