What happens when you over use the sharingan?

Overusing the Sharingan can lead to significant strain on the user's eyes, resulting in a condition known as "Sharingan blindness," where the user gradually loses their vision. Additionally, excessive use may lead to physical and mental fatigue, impairing the user's overall combat effectiveness. In extreme cases, prolonged activation can cause the user to suffer from severe side effects or even permanent damage to their ocular abilities.