Black nails may be caused by gray nails, or trauma or nail moles that lead to black nails, which need to be treated according to the specific symptoms. 1. Gray nails: due to fungal infection of the nails, the nails become cloudy, thickened, brittle, and even the nails gradually turn black and so on. Treatment can be oral antifungal drugs such as itraconazole capsules, topical use of amorolfine application and other antifungal drugs. 2. Trauma: If the nails are traumatized, there may be under-nail bleeding to form bruises, and the nails will turn black in appearance, which may be accompanied by pressure pain. Treatment is mainly to activate blood circulation and remove blood stasis. 3. Nail nevus: it may also turn nails black, it is a kind of black pigmented nevus occurring on nails, its common clinical symptom is a black line appearing on nails. It is a kind of dark pigmented nevus that occurs on the nails. If there is no obvious cause for the nails to turn black, it is recommended to go to dermatology department as soon as possible, and do not treat it by yourself or use medicine to treat it.