Having gray hair at the age of 40 is a normal phenomenon most of the time, but sometimes it can be caused by genetic factors, vitiligo, malnutrition and medication factors. 1. Physiological reasons: with age, long white hair can be a normal physiological phenomenon, can be caused by overwork, mental stress. 2. Pathological causes (1) genetic factors: such as albinism, is a congenital genetic disease caused by defective melanin synthesis, in addition to hair whitening, there will be skin and fundus of the eye hypopigmentation. (2) Vitiligo: mainly manifested as white patches of varying sizes on various parts of the skin. If it occurs in the scalp, the hair can also become white. (3) Malnutrition: If the daily intake of zinc, vitamin B12, folic acid and other nutrients is insufficient, it can also lead to hair malnutrition, growth disorders, and then turn white. (4) Drug factors: tyrosinase inhibitors (such as imatinib, gefitinib, etc.) may also cause hair to turn gray. If, in addition to growing gray hair, other abnormalities occur, it is recommended to visit the hospital in time for standardized diagnosis and treatment.