London girl Sara Harris was out with friends at the annual company Christmas bash. It had been a terrific year - she had the new job, the new car, and great friends. Little did she realise that what she thought was going to be the best night of her life, would turn into a nightmare. Her last drink cost her a six-year jail sentence for drinking and driving offences.

We all like to think we know when we're 'over the limit', and that we understand the impact alcohol has on our bodies, but do we really know how big the risk is? According to UK Department of Transport figures, over 3,000 people are killed or seriously injured every year on the UK's roads in drink-related accidents. In the US it's even more frightening with deaths over 15,000 ! If you're caught drinking and driving, the penalties can be more long-term than you think.

What's your limit?
In the UK alcohol is measured in 'units' of 10 mg, where a unit is equivalent to 8gm or 10ml of pure alcohol. The legal limit immediately before driving is 3 units for women, and 4 units for men. Measurement is similar in the US so the examples we're using apply to both countries.

Do you know what that term "Unit" means in terms of the drink in your hand? If you're a wine drinker, it means only 2 glasses of wine - 1 measure of wine is 1 unit. Spirit lovers can have two shots - 1 measure of spirits equals 1 unit. For beer drinkers, 1 pint of regular strength beer (around 3.6%) is 2 units, so women can have 1.5 pints, and men 2 pints. Remember, of course, that these limits are based on 'pub' (public house) measures.

And what about the morning after? If you plan to drive the day after a big night out, you should keep an eye on your drinking. Women should consume no more than 7 units of alcohol, and men no more than 10, stopping altogether at around 11.30pm if you'll be driving after 8.00am. It takes you body approximately two hours to eliminate 1 pint of beer.

There is, however, no failsafe guide as to how much you can drink and stay under the limit. The effect alcohol has your body varies depending on the type of alcoholic drink, your weight, sex, metabolism and age.

Remember:

• Many beers and wines are stronger than average. For instance a pint of Stella Artois lager would be the equivalent of 3 single shots not 2
• Drinks poured at home are usually more generous than pub/bar measures
• Cocktails and Alcopops are very strong but their content is often masked by the taste of fruit juice
• Even the smallest amount of alcohol will affect your judgment - starting with your ability to judge your alcohol intake!
• At twice the legal limit, you are 50 times more likely to be involved in a fatal collision.

Alcohol's effects on you
You may feel fine, but what is the alcohol doing to your body? Everyone is affected differently by alcohol but the science behind it will not change. After drinking, the brain works inefficiently, taking longer to receive messages from the eyes, processing information becomes more difficult and instructions to the muscles are delayed. Alcohol will slow down reaction times by 10-30 % and reduces the ability to perform two or more tasks at the same time.

So what's the message? Have a careful Christmas and stay safe. The best option is not to drink at all before driving, but if you do, know your limits and stay within them. Remember, if convicted of driving under the influence in either the US or UK you'll have a criminal record, you may not be allowed to drive for at least a year, you could lose your job, your lifestyle will change dramatically, your insurance costs will rocket, and you will have trouble hiring a car for the next 10 years. Let drink driving be a Christmas tradition we can do without this year.