If headache occurs after the fever is reduced, there are two main reasons: 1. The patient may have lost a large amount of water due to sweating when the fever is reduced, and the patient does not replenish it in time, resulting in a decrease in effective circulating blood volume and insufficient blood supply to the brain, causing headache due to ischemia and lack of oxygen to the brain. 2. After the fever subsides, the blood vessels in the brain become constricted, and then the pressure in the blood vessels rises, and the patient develops headache symptoms. In addition, it can also be a symptom of aseptic, reactive meningitis stimulation, and the headache will be significantly relieved after the patient is treated with pain medication.