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Your body may become dehydrated (low on water) for any of a number of reasons. The most common are diarrhea, sweating in hot weather or during strenuous activity such as a marathon race, and water evaporating more quickly at lower air pressure, which is a factor in altitude sickness. The physiology of hangovers is complex, but dehydration is one factor.

Dehydration is always uncomfortable and bad for your health; in extreme cases it can kill.

All of the conditions mentioned above can also involve problems other than dehydration; see the linked articles for details.

Feelings of thirst are an important early symptom of dehydration, and headache is also common. If you are in a situation where dehydration is a possibility and you have a headache, try drinking more fluids and see if it helps. It may not, but it is worth trying.

Needing to urinate after sleeping is normal, and increased urination is one of the body's normal responses to high altitude; if either of those do not happen when expected, consider this a sign of dehydration. Other signs are falling blood pressure, increased pulse rate, and dizziness.

For a simple and reliable sign, check the colour of your urine; the lower your body is on water, the darker it gets. If the urine is distinctly dark or you are not urinating at all, those are signs that you are seriously low on water.

If you do show signs of dehydration – or to prevent it before signs show up – the main thing is to drink enough. You can also try to reduce whatever stress is leading to the dehydration: in hot weather, find air conditioning or at least shade; at altitude, descend or at least do not ascend further; in a marathon, quit or at least slow down.

To maintain hydration, drink enough water or oral rehydration solution. If you have any symptoms of dehydration, drink as much as you can without serious discomfort until the symptoms resolve. A liter an hour is a reasonable target, a glass every few minutes. If you have no symptoms but are at risk of dehydration, drink often.

Other clear liquids – such as weak tea with some sugar, any clear juice, coconut water, or various "sports drinks" – are also good. What American English calls "lemonade" (and other dialects "lemon squash") is fine. Non-clear juices should be diluted with water. Some may find carbonated beverages are easier on the stomach if they are allowed to go flat before drinking. Clear soup or other foods with a high water content such as grapes, watermelon or tomatoes are also good.

Both alcohol and caffeine are diuretics (drugs that increase urination), so booze, coffee, tea and caffeine-containing soft drinks should be avoided or taken in moderation. A glass of beer or a morning cup of coffee are not a problem, but don't overdo it. Avoid hard liquor.

In some cases it may be difficult to drink enough, or to keep it down if you do. Try drinking many small sips. In extreme cases, health care personnel may give fluids intravenously.

If you are losing a great deal of water you also need to replace electrolytes.

• Hyponatraemia. This is a condition of low blood sodium, caused by losing salt by sweating and drinking lots of water without replacing the sodium. It has sometimes been fatal for long-distance runners.  
• Salt tablets are sold mainly in pharmacies. They may be a good idea if you are drinking a lot of anything other than re-hydration solution. An alternative is to just have some salty snacks.
• Salt poisoning. This is a rare condition most often from drinking seawater. It is conceivable it could occur by taking too many salt tablets. Keep the dosage near what is used in rehydration mix – 3 grams of salt per liter of water – and you will generally be safe.  

If you are undertaking an activity such as high-altitude climbing or long-distance running, then you need to have a hydration plan that avoids the risks of both dehydration and hyponatraemia. Creating this may require some discussion with a doctor, preferably a sports medicine specialist.

• Water bottle. You could refill bottles got when buying water. Also more sturdy water bottles are widely available, marketed mainly for hikers, tour cyclists and the like.  
• Hydration pack (drink bag). These are water containers from where water can be sipped through a tube without using your hands. They can be put in your backpack or used as separate backpacks or waist packs, depending on model.  
• Oral re-hydration solution. This is sold in many pharmacies and some expedition shops, either as a liquid or as sachets of powder to be mixed with water.  

You can also make re-hydration solution yourself. One recipe is:

• 1 L (34 fl oz) of water (boiled, bottled, or filtered) to:
• 8 teaspoons (40 mL or 30 g) of sugar and;
• 0.5 teaspoon (2.5 mL or 3 g) of salt, which should be mostly NaCl;

There are a number of additional options including adding half a cup of orange juice or mashed banana to add flavour and help replace potassium. "Saudi champagne", a 50-50 mixture of soda water (club soda) and apple juice, is quite palatable and reasonably close to the right mix for rehydration. Soda water has dissolved salts and minerals and the apple juice has some sugars. However, the carbonation may not sit well on an upset stomach.

You might carry a measuring cup suitable for these quantities, or simply measure a teaspoon and carry that – their volumes vary widely. Alternately, just make up some sugar/salt mix (16:1 by volume or 10:1 by weight) – optionally with a vitamin/mineral supplement pill crushed in – and carry that.

In some countries other salts have been substituted for the essential NaCl in mineral waters, due to worries about excessive salt intake, so check the label.

• Arid region safety
• Hot weather
• Sunburn and sun protection
• Travellers' diarrhea
• Water