A black vertical line in the middle of the toenail may be caused by gray toenail, onychomycosis, trauma and other factors. 1. Gray toenail: It is a lesion caused by infection with skin fungus, which can make the toenail appear white, yellow, dirty color and other changes, or may also make a black vertical line appear in the middle of the toenail. At the same time, the toenail will also appear thickening, separated atrophy symptoms. 2. Nail mole: For example, lack of vitamin B₁₂, systemic pigmentation diseases and other reasons may lead to the occurrence of nail mole. It can cause a black vertical line to appear in the middle of the toenail, and some of them have the risk of becoming cancerous. Therefore, it is recommended to go to the hospital in time when this happens. 3. Trauma: When the toenail is damaged by trauma, it may lead to bruising under the nail, which will lead to the phenomenon of a black vertical line in the middle of the toenail. When encountering a black vertical line in the middle of the toenail, it is recommended to consult a doctor in time to clarify the cause of the disease, which is conducive to targeted treatment.