Red flags: age > 50, worse headache of life, acute/thunderclap onset, new neurological deficits, immunocompromised, known cancer/cancer w/brain met potential, altered mental status, worse w/cough or activity, awakens from sleep, post head trauma. Note that even chronic headaches (tension, cluster, migraine) will have a first time/acute presentation. Also note that acute headache sx that are different than typical HA (for those w/chronic HA) is a red flag.