What causes dehydration?

Dehydration is caused by losing more fluid than you replace, commonly through vomiting, diarrhoea, sweating, fever, urination or not drinking enough. The body also loses salts with some types of fluid loss, which can affect how you feel and how well your muscles and nerves work.

Stomach and bowel infections such as gastroenteritis can cause repeated vomiting or diarrhoea, making it difficult to keep fluids in the body. Fever may increase fluid loss through sweating and faster breathing. Hot weather, strenuous activity and heat exhaustion can also lead to heavy sweating, especially when drinking does not keep pace with the loss.

Dehydration can also follow reduced fluid intake. Nausea, a sore mouth, difficulty swallowing, tiredness, confusion or being unable to reach a drink may all make someone drink less. Certain medicines and health conditions can increase urination, so a clinician may need to review the wider cause rather than treating thirst alone.

Start replacing fluids with frequent small sips. If vomiting makes drinking difficult, pause briefly and try again with smaller amounts. An oral rehydration solution can replace water and salts lost during vomiting or diarrhoea; follow the product instructions for mixing and use. Continue eating light foods when able, and avoid relying on alcohol as a fluid source.

Move to a cool place and rest if heat or exertion triggered the symptoms. Keep monitoring whether drinking becomes easier and whether urination returns to its usual pattern. Do not force large amounts quickly if that worsens nausea.

Seek urgent medical help for confusion, fainting, severe weakness, a seizure, trouble staying awake, or an inability to keep any fluids down. Get prompt advice if there is very little or no urination, worsening dizziness, blood in vomit or stool, severe abdominal pain, or signs of serious heat illness. Babies, older adults and people with significant medical conditions can deteriorate quickly and should be assessed early.

Ask what may be driving the fluid loss, whether an oral rehydration solution is suitable, and whether any medicine could be increasing urination or worsening vomiting. If diarrhoea, fever or heat exposure continues, ask which warning signs should prompt an urgent review.