Black stuff inside toenails may be caused by gray toenails, onychomycosis and other diseases, and it is recommended that patients seek medical examination in time. 1. Gray nails: usually due to the skin fungus invasion of the nail plate caused by nail lesions, if not treated in time, may lead to discoloration of the nail surface, the phenomenon of becoming brittle, may also appear in the toenail inside the phenomenon of black stuff. Patients can use drugs such as amorolfine and cycloheximide for treatment under the guidance of doctors. 2. Onychomycosis: the cause of this disease is not clear, it may be related to environmental heredity and other factors, the occurrence of onychomycosis will lead to the phenomenon of black lines appearing in the toenails of patients. If the patient has no discomfort symptoms, no special treatment is needed. If malignant lesions appear, it should go to the hospital for surgical treatment in time. When there are black things inside the toenail, it is recommended to go to the hospital for examination in time to avoid delaying the condition leading to adverse consequences.