Headache due to dehydration

Fluid loss can follow sweating, heat exposure, fever, vomiting, diarrhoea or simply drinking too little. Headache may occur with thirst, dry mouth, dizziness on standing, fatigue, constipation or darker urine. In hot conditions, headache with heavy sweating, weakness, nausea or cramps may fit heat exhaustion. A urinary infection, kidney stone or gastroenteritis can cause fluid loss or make drinking difficult. Dark urine supports concern about concentrated urine but does not diagnose the cause by itself.

Move to a cool place, rest and take frequent small sips of water. After vomiting or diarrhoea, an oral rehydration solution from a pharmacy can replace fluid and salts; follow the product instructions. Continue light meals if tolerated. Avoid alcohol and intense exercise until you feel recovered. If you have kidney, heart or other conditions that limit fluids, ask a clinician how much is appropriate.

Get urgent help for confusion, fainting, inability to keep fluids down, very little urine, severe weakness, a sudden severe headache or symptoms after significant heat exposure. Arrange prompt care for blood in urine, severe side pain, fever with urinary symptoms, ongoing diarrhoea or a headache that does not improve as hydration returns.

What may be causing my fluid loss? Should I use oral rehydration solution? Do my medicines or medical conditions change my fluid needs?