Migraine: This is often the most common cause of unilateral headaches, including those that are left-sided. Migraines can be triggered by various factors and are typically characterized by throbbing pain, often accompanied by nausea, vomiting, and sensitivity to light and sound.

Tension Headache: While tension headaches are usually bilateral, they can occasionally present unilaterally. They are typically described as a band or a squeezing sensation around the forehead, back of the neck, or both.
Cluster Headache: These are severe, unilateral headaches that occur in clusters or cycles. They are often centered around one eye and can be accompanied by autonomic symptoms like nasal congestion or eye watering.
Sinusitis: Inflammation of the sinuses can cause headaches, which might be unilateral if one sinus is more affected than the others. The pain is usually in the cheek and forehead area.

Temporal Arteritis (Giant Cell Arteritis): Although less common, this condition is critical to diagnose due to the risk of blindness if not treated promptly. It typically presents with unilateral headache, jaw claudication, and visual disturbances in individuals over 50 years old.
Subarachnoid Hemorrhage: A sudden, severe headache (often described as "the worst headache of my life") could indicate bleeding into the space surrounding the brain. This is a medical emergency.
Stroke or Transient Ischemic Attack (TIA): While not typically presenting solely as a headache, in some cases, especially if the headache is sudden and severe, it could be a symptom of a stroke or TIA, particularly if accompanied by other neurological deficits.

Trigeminal Neuralgia: This condition involves sudden, severe, shock-like pain around the eyes, lips, or nose mainly because of abnormal blood vessels pressing on the trigeminal nerve in the head. It's usually unilateral.
Hemicrania Continua: A rare form of headache that is continuous and strictly unilateral, often accompanied by autonomic features like nasal congestion or eye watering on the same side as the headache.
Occipital Neuralgia: Involves inflammation of the occipital nerves and can cause severe, sharp, or stabbing pain in the back and side of the head, typically on one side.