Dehydration (Adults): Dehydration occurs when the loss of body fluids (mostly water), exceeds the amount that is taken in. Causes of dehydration include fever, vomiting, diarrhea, diseases (diabetes), impaired ability to drink, lack of access to safe drinking water, and injuries to the skin. Common symptoms of dehydration include dizziness, dry skin, headache, low urine output, tiredness, sleepiness, dry mouth, and increased thirst. Treatment of dehydration depends upon the severity of the condition.