Does dehydration cause fainting?

Loss of fluid through heat, sweating, vomiting, or diarrhoea may leave the brain briefly under-supplied when someone stands or remains upright. A faint can also result from an abnormal heart rhythm, bleeding, low blood sugar, or a nervous-system response. Because these causes can overlap, do not assume dehydration explains an episode without considering the circumstances.

If you feel faint, lie flat and raise your legs if comfortable; loosen tight clothing and get help. After recovery, take small sips of fluid and avoid standing suddenly. Someone who has fainted should be checked for injury and should not be left alone immediately. Do not drive until a clinician has advised that it is safe.

Call emergency services if the person is not breathing normally, does not wake promptly, has chest pain, severe breathlessness, a seizure, a serious injury, or new weakness or speech trouble. Even after a quick recovery, arrange urgent medical assessment for a first faint, repeated episodes, fainting during exercise, or fainting with palpitations.

Tell the clinician what you were doing, whether you had heat exposure or fluid loss, how long you were unresponsive, and whether there was an injury. Ask if you need blood pressure, blood, or heart checks.