Dehydration and fainting

Fluid loss from sweating, fever, vomiting, diarrhoea or not drinking can lower the volume available to the circulation. Standing suddenly may then bring on light-headedness, blurred vision, nausea or weakness. Fainting can also have causes unrelated to hydration, including heart rhythm problems, low blood sugar or bleeding, so do not assume dehydration is the explanation after a first episode.

Lie flat and raise your legs if possible; turn onto your side if vomiting or if you are not fully alert. Loosen tight clothing and move away from heat. Once fully awake and able to swallow, take small sips of water or an oral rehydration solution. Do not drive, climb or stand quickly until you feel steady. Someone should stay with you after a faint.

Call emergency services for a faint with chest pain, severe breathlessness, a pounding or irregular heartbeat, seizure-like movements, serious injury, confusion or slow recovery. Seek prompt assessment after a first faint, repeated episodes, fainting during exercise, pregnancy, significant bleeding, or ongoing inability to drink. Very little urine and marked weakness also need urgent attention.

Tell your doctor what you were doing, whether you had warning symptoms, how long recovery took, and any medicines, illness, bleeding or family history of heart problems.