What should you know about cellular dehydration?

Water leaves the body through urine, sweat, breathing, vomiting or diarrhoea. When replacement does not keep pace, blood volume and the balance of salts and water can change. Early signs may include thirst, dry mouth, headache, tiredness and passing less urine. [Dark Urine](/symptoms/dark-urine/) can reflect concentrated urine, although food, medicines and other illnesses can also change its colour.

Dehydration can accompany [Gastroenteritis](/conditions/gastroenteritis/) or [Heat Exhaustion](/conditions/heat-exhaustion/). [Constipation](/symptoms/constipation/) may worsen when stool contains less water, but constipation has several possible causes.

Sip fluids regularly rather than waiting for intense thirst. After vomiting or diarrhoea, an oral rehydration solution can replace water and salts; follow the product instructions. Continue suitable foods as tolerated and rest somewhere cool after heat exposure. Infants, older adults and people with kidney or heart problems may need individual fluid advice.

Do not force large amounts quickly if vomiting continues. A clinician can assess pulse, blood pressure, urine output and the likely cause. Cloudy urine is not a reliable hydration gauge and may need assessment for infection or another urinary problem; see [Cloudy Urine](/symptoms/cloudy-urine/) for context.

Seek urgent care for confusion, fainting, severe weakness, inability to keep fluids down, very little urine, rapid breathing or worsening drowsiness. Get prompt advice when dehydration follows persistent diarrhoea, repeated vomiting, heat exposure or a new illness. Pain in the side with urinary symptoms may need assessment for [Kidney Stones](/conditions/kidney-stones/) or a [Urinary Tract Infection](/conditions/urinary-tract-infection/), rather than fluids alone.

Ask: How dehydrated am I? Which fluid and salt replacement is appropriate? Could a medicine or illness be causing the fluid loss? How should I adjust fluids if I have kidney, heart or liver disease?