Headache red flags are clinical features that suggest a secondary (potentially dangerous) etiology requiring urgent investigation. The SNOOP mnemonic provides a systematic screening framework. Systemic symptoms (fever, weight loss, night sweats) or Secondary risk factors (HIV, cancer, immunosuppression) raise concern for meningitis, brain abscess, or CNS lymphoma/metastases. Neurological deficit or dysfunction (focal weakness, ataxia, papilledema, seizure, confusion) suggests a structural lesion causing mass effect, herniation, or cerebrovascular event. Onset sudden (thunderclap headache reaching peak intensity within seconds) is the hallmark of subarachnoid hemorrhage from ruptured berry aneurysm (most commonly at the anterior communicating artery), but also occurs in reversible cerebral vasoconstriction syndrome (RCVS), cerebral venous sinus thrombosis, pituitary apoplexy, and cervical artery dissection. Older age of onset (new headache in patient >50) raises concern for giant cell arteritis (GCA — granulomatous vasculitis of medium/large temporal arteries causing transmural inflammation, vessel wall thickening, and potential anterior ischemic optic neuropathy leading to irreversible blindness) and intracranial neoplasm. Pattern change (new daily persistent headache, progressively worsening headache, headache worst on waking, headache worsened by Valsalva or positional changes) suggests raised intracranial pressure from...