Sudden hair loss may be caused by baldness, drug alopecia, endocrine factors, mental factors and other reasons. 1. Pemphigus vulgaris: the main manifestation of pemphigus vulgaris is the sudden appearance of round or oval patchy hair loss. 2. Drug alopecia: drug alopecia, such as the side effects of chemotherapy drugs for malignant tumors, can cause massive hair loss. 3. Endocrine factors: endocrine factors such as female postpartum alopecia, most of the symptoms of sudden hair loss in the postpartum 2~6 months. 4. Mental factors: mental factors such as excessive mental stress, tension, anxiety, etc. may also cause sudden hair loss. Sudden hair loss may also have other reasons, it is recommended to go to the hospital in a timely manner to improve the examination, to clarify the cause of the disease, and then give targeted treatment.