One of the main downsides of the holiday season is the hangovers that inevitably follow the celebrating. BBC News Online explains what they are, how to avoid them and what to do if you do fall foul of the morning after blues.

The Christmas season is always a round of parties, but this year the millennium celebrations will add to the social whirl. The only drawback to all this good cheer is the hangovers that are bound to follow when you over-indulge.

The question is, short of giving up the drink altogether, how do you avoid hangovers in the first place, and what can you do to lessen their effects?

Approaching the drinking season with a bit of scientific knowledge can prove invaluable. The first thing to remember is that alcohol, or ethanol as it is in its pure form, is a diuretic - a substance that encourages urination, leaving the drinker dehydrated because the body loses more water than it takes in.

The body reacts to this lack of water by taking it from other areas, such as the brain - one reason why hangovers are associated with headaches.

The answer to this may as a result be to consume water at the same time as alcohol, to ensure the body's fluid stocks are kept topped up.

Indeed, Griffith Edwards, professor of addiction behaviour at the National Addiction Centre in London, says this is one of the few things which does you any good when you drink.

"Dehydration alters all sorts of pressure relationships associated with headaches, and dehydration can cause hangovers. You may be able to head it off with water the night before," he said.

But once the hangover has taken hold there is very little you can do, he added. "Try some more water, otherwise I think you grin and bear it.

"Sales of hangover cures go up at this time of year, but they are mainly placebos, they don't really work," was his uncompromising message.

A survey of hangover cures carried out by New Scientist magazine went along with this view up to a point. Tests by the magazine's authors found that water had little effect, as did "sports drinks", which some believe are useful because they contain high levels of vital ions and sugars - also lost when drinking alcohol.

Mop up

They did, though, give a warm response to N-acetyl-cysteine - an amino acid supplement sold in health food stores, thought to work by increasing the body's ability to mop up destructive chemicals which build up in the liver as enzymes break down ethanol.

Its apparent success also pointed to the reasoning behind some traditional hangover cures, such as raw egg and fry ups, eggs also being rich in cysteine.

Some experts, though, warn against another commonly taken "cure" - paracetamol, which the US Food and Drug Administration says amplifies the effect on the liver.

But Professor Edwards says one of the key causes of hangovers is not the ethanol itself, but the added toxins which give different drinks their distinctive flavours. And he makes an important distinction between hangovers and withdrawal symptoms - shaking, sweating, nausea and anxiety - all of which can be signs of an alcohol dependency.

The underlying message is that moderation is the only real answer to avoiding the festive season being more about illness than enjoyment.