Black toenails are caused by trauma to the nail. Common causes of toenails turning black include ill-fitting shoes, fungus, dropping an object on the toe and cutting the nail too short. Impact to the toenail causes bleeding underneath the nail's surface. Part or all of the toenail may turn black. Subungual hematoma is the medical term for black toenail, literally meaning "blood underneath a nail."
Athletes often suffer from black toenails, especially those who run on hills or play soccer. A shoe without enough space for the longest toe, usually one thumbnail length, is to blame. When the toe rams into the shoe or the foot slides around, blood is pushed back under the nail with each step or kick. Athletes can wear socks that wick away sweat to prevent sliding.
Clipping toenails straight across at a moderate length lessens the chance of ingrown toenails, which can lead to black toenails. When the sides of the nail dig into the skin, folds cover the nail as it grows. Tight-fitting shoes can also cause ingrown toenails. The pressure can break the skin, leaving it open to infection and discoloration from blood.
Another cause of black toenails is fungus, or onychomycosis. This common fungus begins with white spots and can advance to thick, yellow toenails. Again, ill-fitting shoes rub tiny breaks into the nail, leaving it open for infection from foot perspiration. Typically affecting the big toe, this fungus puts pressure on the toe, causing painful inflammation.
Athletes are at risk for fungus because it thrives in a warm, moist shoe. The elderly and diabetics are also at risk. Mild fungus is easily treated with an antifungal, over-the-counter oral medication. Prescription topicals are available at a higher cost, but may be a safer choice for diabetics with a moderate case. More advanced, repeated cases may require laser treatment, where a laser kills the pathogens which started the infection.
Black toenails are mostly harmless. Sometimes, in painful situations, the liquid underneath the toenail needs to be drained or the nail removed. Often, however, black toenails simply fall off and a new toenail grows in.
A rarer cause of a discolored toenail is melanoma. Malignant melanoma in the foot is easily treated if diagnosed early. In instances of trauma, the discoloration is underneath the toenail and moves with the nail as it grows. If no trauma has occurred and no fungus is present, a podiatrist should evaluate the nail soon after discoloration appears. The doctor may request a biopsy to confirm suspicions.