Most headaches are caused by one of the factors outlined previously. In some rare cases, though, headaches are symptoms of serious conditions. If a headache comes on suddenly and is blindingly painful, it could be caused by a brain aneurysm. This symptom has been described as "the worst headache imaginable," so it should be easy to distinguish from a regular headache.
If you develop a sudden, extremely painful headache, it is crucial to seek immediate medical attention, as a brain aneurysm is life-threatening.

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