Tension Headache: This is often the most common cause of headache and can present with red flag symptoms if severe or persistent, but it's essential to rule out other more serious causes.

Migraine: Migraines can cause severe headaches and may be accompanied by red flag symptoms such as aura, nausea, and sensitivity to light and sound.
Cluster Headache: Known for their severe, unilateral pain, cluster headaches can be mistaken for other red flag conditions due to their intensity and associated symptoms like nasal congestion and eye watering.
Sinusitis: Inflammation of the sinuses can cause headache, especially in the frontal area, and may be considered a red flag if symptoms like fever, facial pain, and nasal discharge are present.

Subarachnoid Hemorrhage (SAH): A life-threatening condition caused by bleeding into the space surrounding the brain, often presenting with a sudden, severe headache (thunderclap headache).
Meningitis: Inflammation of the protective membranes covering the brain and spinal cord, which can present with headache, fever, and stiff neck.
Temporal Arteritis (Giant Cell Arteritis): An inflammation of the blood vessels, particularly those in the head, which can lead to severe headache, visual disturbances, and jaw claudication.
Brain Tumor: Although rare, brain tumors can cause headaches that worsen over time, especially if they increase intracranial pressure.

Cervicogenic Headache: Originates from the cervical spine and its structures, often due to trauma or degenerative changes.
Trigeminal Neuralgia: A chronic pain condition that affects the trigeminal nerve, causing intense shock-like or stabbing pain in parts of the face.
Pituitary Apoplexy: A rare but potentially life-threatening condition caused by hemorrhage or infarction of the pituitary gland, presenting with sudden, severe headache and visual disturbances.
Idiopathic Intracranial Hypertension (IIH): Characterized by increased pressure within the skull without a detectable cause, often presenting with headache and visual symptoms.