Toenails become black, thick and painful, and are considered to be related to factors such as gray nails combined with onychomycosis and melanoma. It is recommended to make a clear diagnosis, give appropriate medication, and treat with surgery if necessary. 1. Gray nails combined with onychomycosis: Gray nails is a nail fungus disease caused by fungal infection. Fungus invades the nail plate and nail bed, resulting in abnormalities of the nails, such as discoloration and thickening, shedding and deformation. If combined with onychomycosis, it will lead to local redness, swelling and pain. Anti-fungal medications such as amorolfine rub and cyclopiazone nail coating can be used as prescribed by the doctor. If necessary, oral itraconazole, fluconazole and other antifungal drugs. 2. Melanoma: long black toenails accompanied by pain can also be considered related to melanoma. Gradual increase of melanoma can lead to localized blackening, accompanied by obvious pain, can go to the hospital for examination, if necessary, through surgical excision treatment. Toenails become black and thick, accompanied by pain, but also to understand whether it is related to the role of external forces, it is recommended to go to the hospital for examination to clarify the cause.