Dehydration and tiredness: what to know

Fluid supports blood flow, temperature regulation and the delivery of oxygen and nutrients around the body. Losing fluid through sweating, vomiting, diarrhoea or not drinking enough can leave you drained, dizzy or headachy. Dry mouth, thirst, dark urine and passing urine less often may occur alongside fatigue.

Tiredness is not specific to dehydration. Poor sleep, infection, anaemia, medication effects and other health problems can also cause it. If fatigue continues after drinking normally, or keeps returning, a clinician can help look for another cause.

Rest somewhere cool and sip fluids regularly rather than drinking a large amount quickly. If diarrhoea or vomiting is involved, an oral rehydration solution can replace water and salts. Eat light foods when you can, and avoid alcohol until you feel recovered. Notice whether alertness, thirst and urine output improve; do not drive if dizziness or marked weakness is present.

Seek urgent care for confusion, fainting, severe dizziness, a seizure, inability to keep fluids down, very little or no urine, or unusual drowsiness that is getting worse. Arrange medical advice when tiredness lasts despite rehydration, follows ongoing fluid loss, or occurs with chest pain, breathlessness, blood in stool, severe abdominal pain or a persistent fever.

Ask what may be causing the fluid loss, whether you need an oral rehydration product, and which symptoms should prompt same-day assessment. Mention your medicines and any kidney, heart or endocrine condition.