Unsurprisingly, the answer is fairly complex. If there was a single simple reason for all that pain, you can be sure that a catch-all cure would have been discovered long ago. As it is, there is a veritable cocktail of effects, all of which conspire to make 'the morning after' a grim one.
Pure ethanol is metabolically fairly clean, but an alcoholic beverage is a combination of water, ethanol, and flavourings, and it's the identity of some of these flavourings that causes many of the problems. Red wines contain all sorts of interesting chemicals, leading to the complex flavourings typical of the breed, and although many of these impurities - such as arsenic - are poisonous, they are usually present in such minute quantities as to be relatively harmless. However, if the wine is concentrated by distillation, then as well as increasing the alcohol content, you are also concentrating the poisons. This is the reason that brandy, port and cheap red wine can give you the most monstrous hangovers, as well as gout in later life.
Dehydration is a well-documented consequence of drinking and is caused because ethanol has diuretic qualities, so you end up expelling more liquid than you drink. This also results in the loss of important salts dissolved in it. Potassium and sodium ions in particular are essential for the optimal functioning of nerves and muscles. An imbalance outside a limited range can result in nausea, fatigue, and headaches.
Ethanol also acts on the brain's pituitary gland and blocks production of a hormone called vasopressin, which usually directs the kidneys to reabsorb water that would otherwise end up in the bladder. Once this hormonal signal has been switched off, there is nothing to stop the bladder from filling up with all the water from the fluid that you drink. A supply of water is essential to the continuing functioning of the body, and when various organs find that their normal supply of water has been cut off, they steal it from anywhere they can, including the cells of the brain. Although the brain itself cannot feel pain, when it starts to shrink due to water loss, pain-sensitive filaments connecting the outside membranes to the inside of the skull become stretched, giving the symptoms of a headache.
Alcohol also attacks the body's store of glycogen, an important energy source kept in the liver, breaking it down to glucose which is then flushed out in the urine. Without this store of energy on call next morning, you are left feeling weak and wobbly.
Methanol is a simpler cousin of ethanol, and is found as a contaminant in cheap red wines, whisky and fruit brandy. This is 'meths', the fuel alcohol that makes you blind if you drink too much of it. The liver attacks it as the poison it is, but one of the break-down products is formic acid, a particularly nasty chemical which ants use to spray at their attackers.
But anyway, enough of the reasons why you're feeling rubbish and onto the cures...
1) Over the counter remedies
Sold in Australia as a vitamin supplement, Berocca is widely recognised as a hangover cure as it contains all the chemicals that are lost and destroyed in a drinking session, in the correct proportions.
N-acetyl-cysteine (NAC) is an amino acid supplement sold in health food stores, and is extremely good at mopping up the free radicals that have built up in the liver. NAC works because it is rich in cysteine, another amino acid that is used by the body in the manufacture of free radical-eliminating glutathione. For those in the know, this is a very effective hangover remedy, and is especially good if you need a clear head in the morning.
Sold in the UK as a stomach settler, Resolve is a powder that becomes a fizzy drink when added to water, and contains a painkiller and some anti-acid chemicals. Another common brand, although without the painkiller, is Alka Seltzer which comes in tablet form. For our purposes, these are best taken before going to bed, as the chances are that in the morning you won't be able to keep it down. It can work marvels, especially if followed in the morning by a vitamin supplement such as Berocca.
Many traditional hangover cures, such as Prairie Oysters, omelettes and the English Fried Breakfast, involve eggs. Others swear by a downed raw egg in the morning. The reason that these are thought to work at all is probably that eggs also contain cysteine, and so help to mop up free radicals.
3) A hot shower
Another way of relieving a headache is to sit in a really hot, really powerful shower, and get the full force of it on the back of your neck. This may need some juxtaposition of plastic chairs and shower settings, so it might be an idea to practice first while sober, but it is worthwhile because headaches are often caused by constricted blood vessels and tense neck muscles. A massage under a hot shower relaxes the tension.
4) Isotonic sports drinks
In theory these are a great idea, for they are supposed to replace all the salts and sugars that are sweated out during athletic activity - surely much the same thing as we are trying to achieve here. The problem is that, due to market forces, they are usually fizzy, and probably the last thing you need while suffering a hangover is a bellyful of bloating gas. However, if you don't mind, or if you can find a flat one, it's definitely worth doing. One variation on the sports drink theme is a 50:50 mix of Tropical Tango and Red Bull.
5) Salt Solution
Apparently a poisoned digestive system is much better at taking up an isotonic solution than it is at taking up pure water, so if you're going to drink water put a spoonful of salt in it, and a couple more of sugar to increase the concentration and mask the taste. While you're at it, you might as well throw in some powdered painkillers, although bear in mind that some studies have shown that paracetamol can amplify alcohol's damaging effect on the liver. As with any medication, read the packet carefully.
6) A breath of fresh air
Popular wisdom dictates a brisk walk in fresh mountain air to dispel those post-binge blues, but the problem is that when you really need it, the last thing you're capable of doing is getting up off the floor, let alone going out into the outside world. The theory is that the increased oxygen flow improves the metabolic rate, and thus increases the speed at which the poisons are broken down. Be that as it may, SCUBA divers have long known that a blast from the tank first thing in the morning does wonders in blowing away the fog.
7) Pinching your hand
There is a nerve junction between the thumb and forefinger on your left hand which is reputed to be an acupressure point which can release tension in the head and neck. If you pinch it quite hard for 30 seconds every five minutes, normal tension headaches can be relieved. It's certainly worth trying if you can't keep down any painkillers.
And if you're really desperate there's always...
8) Kidney dialysis
Since you cannot depend on your kidneys to filter your blood properly after a binge, you could get a machine to do it for you. Admittedly most people don't have access to a dialysis machine, but if you can stand getting hooked up by nurses armed with needles while still drunk, you can be sober in four or five hours without any ill effects. Marvellous.
The following are also all commonly held to be effective hangover cures, but the truth of the matter is that they're just unhelpful myths, so I would recommend giving the following a miss in 2010...
A good strong cup of coffee or tea will perk you up at any time of day, but that's just the caffeine stimulating your tired body. It doesn't actually cure anything, and if you're at the stage when you can keep hot drinks down then you're probably on the road to recovery anyway. In addition, caffeine is also a diuretic, and you don't want to be losing any more water at this stage of the game, so from this viewpoint it may be best to avoid caffeine.
2) Hair of the dog
A tot of alcohol in the morning. For some particularly nasty hangovers, this can be useful, although the bad news is that the effect is only temporary. The liver attacks poisons in a certain order, with ethanol first. Once all the ethanol has been broken down, it starts on the methanol, which releases formic acid into your system and makes you feel bad. Hitting the liver with another dose of ethanol causes it to stop processing methanol and start on the new threat, but the methanol will have to be processed sometime so you are only postponing the hangover until later.
The traditional hangover remedy, with folklore dictating that you should quaff a pint of water for every drink that you have consumed. Undeniably this has some ameliorative effect, but because your kidneys' water-absorption function has been switched off, a lot of it goes straight to your bladder, noticeably causing nocturnal trips to the bathroom and little else.
Please note that we take no responsibility for any ill effects caused by the remedies suggested here: you try them at your own risk. Nobody should mix their medicines. Similarly nobody should drink ten pints of lager and then eat a curry. You have been warned!