What should you know about consequences of dehydration?

When the body loses more fluid than it replaces, the heart has less circulating fluid to move. This may cause thirst, dry mouth, tiredness, headache, dizziness and reduced concentration. Urine can become darker and less frequent. Ongoing fluid shortage makes stool harder and may contribute to constipation; it can also make urine more concentrated, which can support kidney stone formation in some people. Dehydration may occur with vomiting or diarrhoea from gastroenteritis, heavy sweating during heat exhaustion, fever, or difficulty drinking.

If you are awake and able to drink, take small, frequent sips of water. An oral rehydration drink may help when fluid loss follows vomiting or diarrhoea because it replaces water and salts. Rest somewhere cool, pause strenuous activity, and avoid alcohol while recovering. If nausea makes drinking difficult, try small sips rather than a large glass. A clinician can advise on fluid replacement if you have kidney, heart or other conditions that affect how much you should drink.

Seek urgent medical help for confusion, fainting, severe weakness, inability to keep fluids down, very little or no urine, or worsening dizziness. Get prompt assessment if dehydration follows prolonged vomiting, diarrhoea, heat exposure or an illness, or if you notice severe side or back pain that could reflect a kidney stone. Children, older adults and people with chronic illness can worsen quickly, so arrange medical advice early when drinking is not keeping pace with fluid loss.

Ask what may be causing the fluid loss, how much and what type of fluid is suitable for you, and whether medicines or an underlying illness could be contributing. You can also ask which urine changes, bowel symptoms or dizziness should prompt a same-day review.