Short answer: A new, extremely severe headache that reaches peak intensity within about 1 to 5 minutes is a thunderclap headache and needs emergency assessment. It does not automatically mean bleeding in the brain, but symptoms alone cannot rule out subarachnoid haemorrhage or another serious cause. Call your local emergency number promptly, even if the pain begins to ease.
What makes this headache different?
The key feature is the abrupt onset and rapid rise to maximum intensity, rather than pain severity alone. NICE describes a sudden severe headache that typically peaks within 1 to 5 minutes as a red flag for subarachnoid haemorrhage. If you have a history of migraine, do not assume a headache with a distinctly different onset is your usual migraine.
Why is urgent assessment needed?
Subarachnoid haemorrhage is bleeding around the brain and is a medical emergency. A sudden, severe headache may occur with neck stiffness or pain, vomiting, light sensitivity, confusion, fainting, seizure, visual changes, weakness or speech difficulty. These additional symptoms increase concern, but their absence does not make a thunderclap onset safe to dismiss. A clinician will assess the onset, examination findings and need for testing.
What should you do now?
If a severe headache reached its peak within minutes, contact emergency services rather than waiting to see whether a painkiller works. Do not drive yourself. Seek immediate help especially if there is fainting, confusion, a seizure, a new neurologic symptom, severe vomiting or a stiff neck. Tell the team about any recent head injury and medicines you take, including anticoagulants or antiplatelet drugs. Do not change prescribed medicine solely on the basis of this article.
What information helps the clinical team?
Note the time the headache began and how quickly it became most severe, what you were doing, associated symptoms, and how this episode differs from previous headaches. Do not delay the emergency call to complete a record. Useful questions after assessment include: “What urgent causes are being considered?” and “Do the test results and symptoms call for further evaluation?”
Related guides
- Slurred speech that went away
- Symptom timeline and visit questions
- Health records and visit preparation
Sources and review date
- NICE guideline NG228: Subarachnoid haemorrhage — thunderclap onset and urgent evaluation
- NHS: Subarachnoid haemorrhage — symptoms and emergency action
- NLM MedlinePlus: Subarachnoid hemorrhage — causes, symptoms and assessment
Reviewed October 11, 2026. Use your local emergency number: 119 in South Korea, 999 in the UK, or 911 in the US. This is general information and does not replace individual medical care.