Not drinking enough fluid can lead to dehydration, which affects almost every system in the body. How quickly it happens depends on heat, activity, illness, medications, age, and how much fluid you’re losing.
What can happen
You become thirsty and develop a dry mouth — often among the earliest signs.
Your urine becomes darker and less frequent because the kidneys conserve water.
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nhs.uk
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Headaches, tiredness and difficulty concentrating can occur.
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University Hospitals of North Midlands
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Dizziness or light-headedness may develop, particularly when standing up, as blood volume falls.
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NIDDK
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Constipation can worsen because inadequate fluid makes stools harder to pass.
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NIDDK
Your heart and circulation can be affected. More significant dehydration can contribute to low blood pressure and a rapid heartbeat.
Your kidneys have to work harder. Prolonged or severe dehydration can contribute to kidney injury.
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Leeds Teaching Hospitals NHS Trust
Severe dehydration can cause confusion, fainting, shock and organ damage and can become life-threatening if untreated.
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NIDDK
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How much should you drink?
There’s no single amount that’s right for everyone. Fluid needs increase with hot weather, exercise, fever, vomiting or diarrhea. Water from foods and other beverages also contributes to your total fluid intake.
A practical sign is your urine: pale yellow generally suggests adequate hydration, while persistently dark urine and reduced urination can indicate that you need more fluid.
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Royal Devon NHS
If you have kidney failure, heart failure, liver disease, or have been prescribed a fluid restriction, don’t simply increase your water intake—follow your clinician’s instructions.
Seek urgent medical care for severe dehydration accompanied by confusion, difficulty waking, fainting, very little/no urine, rapid breathing, or difficulty breathing.
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