What should you know about hypotonic hydration?

Water and low-electrolyte drinks are hypotonic compared with blood. They can be useful after ordinary fluid loss, but heavy sweating, vomiting or diarrhoea also removes salts. Replacing only water in large amounts can upset the balance between water and sodium. Dehydration may occur with thirst, dry mouth, dark urine, tiredness or dizziness; dark urine is a useful clue but does not identify the cause.

The risk is greater for people with kidney, heart or hormone conditions, or for anyone taking medicines that alter fluid balance. Gastroenteritis and heat exhaustion can both require a more tailored rehydration plan.

Drink regularly according to thirst and replace fluid losses gradually. After substantial vomiting, diarrhoea or sweating, ask a pharmacist or clinician whether an oral rehydration solution is suitable rather than relying on plain water alone. Avoid forcing large volumes quickly. Note urine colour, dizziness, vomiting and how often you are passing urine. People with kidney or heart disease should get individual advice before changing fluid intake.

Seek urgent medical help for confusion, fainting, seizures, severe weakness, trouble breathing, inability to keep fluids down or very little urine. Arrange prompt assessment for persistent vomiting or diarrhoea, worsening dizziness, marked thirst with frequent urination, or symptoms after significant heat exposure. These signs can reflect dehydration, an electrolyte problem or another illness and need assessment rather than self-treatment.

Ask whether your symptoms fit dehydration, whether blood sodium or kidney tests are needed, and which drink is appropriate for the fluid loss. Mention your medicines and any history of kidney stones, urinary infection or heart disease.