What are the first signs of dehydration?

Thirst is a useful warning, but it may not appear promptly in young children, older adults, or people who are unwell. A change in urine colour or frequency can provide another practical clue.

Dehydration happens when the body loses more fluid than it replaces. Fluid loss may follow sweating, fever, vomiting, diarrhoea, heavy exercise, or simply drinking too little. As fluid falls, the body makes less urine and concentrates it, so urine may look darker and smell stronger.

Dry lips, a sticky mouth, low energy, light-headedness when standing, and muscle cramps can also occur. These symptoms are not specific to dehydration, so consider the surrounding circumstances and whether they improve after fluids.

Start replacing fluids with small, frequent sips of water. If vomiting or diarrhoea has caused fluid loss, an oral rehydration solution can replace water and salts; follow the product directions. Rest somewhere cool and loosen excess clothing if heat or sweating contributed.

Continue checking how often you pass urine, its colour, and whether alertness and dizziness improve. Avoid relying on alcohol as a fluid source. If you have a medical condition that limits fluids, ask a clinician how much to drink.

Seek urgent medical help for confusion, fainting, unusual sleepiness, severe weakness, inability to keep fluids down, very little or no urine, or rapid breathing. These can signal significant fluid loss or another serious problem.

Contact a clinician if symptoms continue despite drinking, follow repeated vomiting or diarrhoea, occur with a high fever, or affect a baby, older adult, or person with a long-term illness. Dizziness that is severe, sudden, or associated with chest pain or trouble breathing needs urgent assessment.

Ask what may be causing the fluid loss, whether an oral rehydration solution is suitable, and which warning signs should prompt same-day care. Tell the clinician when symptoms began, how much you have been drinking, whether you have been vomiting or passing loose stools, and what medicines you take.