Fingernails with vertical black lines may be caused by trauma, nail diseases such as onychomycosis and other systemic diseases. 1. Trauma: If the nails are subjected to external impact, extrusion and contusion, etc., there will be a certain amount of bruising within the nails, which will lead to the appearance of black vertical stripes in the process of growth, which generally do not require any special treatment, and can be self-healing after a period of time. But if the pain is unbearable, you can go to the hospital for diagnosis and treatment. 2. Nail diseases: some diseases such as onychomycosis can be manifested as black vertical lines on nails, onychomycosis, nail dystrophy can also appear black vertical lines. Among them, nevus of onychomycosis needs attention, if the black vertical lines are fuzzy and spreading, you need to go to the hospital for consultation, ringworm of the nail can be used under the guidance of the doctor, such as itraconazole and so on. 3. Other diseases: the adverse reaction of drugs, the use of certain tumor chemotherapy drugs (such as doxorubicin), containing psoralen similar drugs (such as trimethoprim) and so on can appear longitudinal black nails, this time you need to ask the doctor to assess whether to continue to use the drug. For specific cases, it is necessary to go to the hospital and be treated under the diagnostic guidance of the doctor.