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Dehydration is a condition caused by an insufficient amount of water in the body. Outside of hospitals, it's generally the result of lack of water, such as might occur when a person is stranded in an arid environment. However, it is not uncommon in hospitals with patients with several common conditions being at risk. The three main causes are:

Water is not recommended for patients suffering from severe dehydration as they may suffer from water based edema in the digestive system as it tries to absorb the water. There are special electrolyte solutions for such situations which mix glucose and salts with the water to allow even absorption of the water.

Unconscious patients are usually given intravenous fluids to avoid dehydration. However, the urine production of these patients must be closely monitored to ensure that they have sufficient kidney function, otherwise edema will result and dialysis may be necessary.

Dehydration at Wikipedia