A headache on the left side may be a sign before the onset of a primary headache, brain hemorrhage, eye disease, temporal arteritis, or other diseases. It is also possible that it is not a disease, but an irregular routine and work, resulting in physiological headaches. 1. Physiological factors: If the daily routine is not regular, work pressure, may trigger the left side of the headache, usually regular work and rest time, avoid staying up late for a long time, appropriate relaxation, headache phenomenon can be relieved. 2. Pathologic factors: (1) Primary headache: the pain caused by the disorder of brain nerve function regulation and vasodilatation dysfunction, primary headache may be related to menstruation, diet, insomnia, medication and other factors, and the patient may also have nausea and other symptoms. (2) Cerebral hemorrhage: after the blood enters the ventricular system, it stimulates the patient’s meninges, thus causing headache, and the patient sometimes has vomiting, numbness and other uncomfortable symptoms. (3) Eye diseases: If the patient suffers from eye diseases such as glaucoma, it may also cause left side headache. (4) Temporal arteritis: Left side headache may also be a precursor of temporal arteritis, and may be accompanied by fever. If the left side of the headache, after excluding the physiological factors, should be timely consultation, clear reasons for targeted treatment.