Tension Headache: This is the most common type of headache and can present unilaterally, including on the left side. It is characterized by a dull, aching pain and is often associated with muscle tension in the neck and scalp.

Migraine: Migraines can cause severe, unilateral headaches, often on one side of the head, including the left. They are typically accompanied by other symptoms such as nausea, vomiting, and sensitivity to light and sound.
Cluster Headache: Although less common, cluster headaches can occur on one side of the head and are known for their severe, piercing pain. They often occur in clusters or cycles and can be accompanied by nasal congestion and eye watering on the affected side.
Sinusitis: Inflammation of the sinuses can cause headaches, which may be unilateral if the sinuses affected are primarily on one side of the head. The pain is often accompanied by facial pressure, nasal congestion, and discharge.

Subarachnoid Hemorrhage: A life-threatening condition caused by bleeding into the space surrounding the brain. It can present with a sudden, severe headache, often described as "the worst headache" ever experienced. Although it may not always be unilateral, it's crucial to consider in the differential diagnosis due to its high morbidity and mortality.
Temporal Arteritis: An inflammation of the blood vessels in the head, which can cause unilateral headaches, typically in older adults. It's essential to diagnose promptly to prevent complications such as vision loss.
Stroke or Transient Ischemic Attack (TIA): While less common as a cause of headache, strokes or TIAs can present with unilateral headache among other neurological deficits. Early recognition is critical for effective treatment.

Trigeminal Neuralgia: A chronic pain condition that affects the trigeminal nerve, which can cause intense, shock-like or stabbing pain in parts of the face, including the area supplied by the left trigeminal nerve.
Occipital Neuralgia: Involves the occipital nerves and can cause unilateral headaches, typically starting at the base of the skull and radiating to the sides of the head.
Hemicrania Continua: A rare form of headache disorder characterized by a continuous, unilateral headache with periods of exacerbation. It's often accompanied by autonomic symptoms like nasal congestion and eye watering on the side of the headache.