Black toenails may be caused by onychomycosis, trauma, melanoma, etc. It can be treated with medication and surgery. 1. Onychomycosis: Fungal infection causes inflammatory reaction of nails as well as nail bed, which may cause fragile texture of toenails, thickening of toenails, and change of toenail color, and may lead to black toenails and other symptoms. You can use topical amorolfine rubbing agent, cyclopyrone rubbing agent, oral fluconazole capsules, terbinafine tablets and other drugs. 2. Trauma: local extrusion, trauma or women wearing high-heeled shoes for a long time, causing excessive local pressure, subcutaneous capillary rupture may also cause black toenails. Generally no special treatment is needed, and most of them will recover gradually with metabolism. 3. Melanoma: it is a kind of malignant tumor originated from melanocytes, which may also cause the symptom of black toes. It needs to be removed by surgery as soon as possible. Black toenails, there may be other reasons, you need to go to the hospital for examination, clear cause of the treatment as prescribed by the doctor. All of the above medications should be used in accordance with medical advice.