Injury and fungi account for most black toenails. Although a black toenail may look unsightly, it rarely causes any serious damage. Damaged toenails repair themselves by growing out, but toenails grow slowly, so don't expect immediate improvement. Only rarely do black toenails have a serious cause that needs medical evaluation.
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If you stub your toe, drop something on it, or wear shoes that rub the nail the wrong way, the nail may turn black in the area of injury. Blood underneath the nail, medically termed a subungual hematoma, looks like a black spot under the nail. If the area continues to bleed, it can cause severe pain relieved by making a small hole in the nail. Call your doctor before trying this yourself. The spot will grow out as the nail grows. In some instances, you may lose the nail after trauma severe enough to cause the nail to turn black.
A nail fungus, also called onychomycosis, can cause a darkened nail as well as a yellowish discoloration, crumbling, distorted or brittle nails. Fungi enter under the corner of the nails and grow best in dark warm, moist environments, such as shoes. Fungi affect toenails more often than fingernails because blood doesn't circulate as well in toes as in fingers, so your immune system has a harder time detecting and attacking the fungi. Fungal infections are difficult to treat and often recur. Over-the-counter creams don't work particularly well and you need to take oral medications for six to 12 weeks, MayoClinic.com states. Antifungal medications taken by mouth could also cause liver damage in susceptible people. Treatment with laser after an acid treatment may help, but not all practitioners offer this treatment.
Acral lentiginous melanoma, a type of skin cancer, can cause a black spot or streak under the toenail. This type of cancer, which comprises 5 percent of all melanomas, often appears on the big toes, according to SkinCancerNet. If you have a nail steak or spot that doesn't grow upward as the toenail grows, if a spot enlarges or if the nail separates from the bed, see a dermatologist.
Wearing properly fitting shoes and wearing shoes when you're working with tools or in other situations where you could drop things on your feet help reduce the risk of accidental injury. Reduce the risk of developing nail fungus by wearing foot coverings in public places and letting your feet "air out" by taking your shoes off occasionally during the day. Choose synthetic rather than cotton or wool socks and change your socks frequently. Report any black spots that enlarge or that don't grow out promptly to your doctor, so he can check for possible melanoma.