Even mild dehydration narrows blood vessels and reduces blood volume to the brain, triggering headaches that range from dull to migraine-like. The fix is usually simple: 2–3 glasses of water and electrolytes, with improvement in 30 minutes to 3 hours. If you're too nauseated to drink or the headache is severe and unusual for you, come in — we can give anti-nausea medication, guide rehydration, and rule out other causes.