The good news: As long as you're not in pain, it will probably heal on its own.

By Roshini Rajapaksa, MD
August 23, 2016
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Q: One of my toenails is turning black-and-blue from running. Should I be concerned?

A: Bruising or slight bleeding beneath a nail is a common runner's injury. Your toe may be bumping against the inside of your sneaker as you run, creating a blood blister under the nail. Check if your shoes are the right size, either by visiting a special running shoe shop or by measuring the space between the tip of your longest toe and the end of the shoe—you want a thumb's width of room. Consider reducing the amount of running you're doing or lessening the intensity so that your toenail can heal without repeated stress.

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As long as you're not in pain, leave the nail alone. It may eventually fall off, and a new one will grow. If your toe is throbbing and sensitive because of an accumulation of blood under the nail, your doctor may need to make a hole in the nail to relieve the pressure. Do not try to do this by yourself; it could lead to an infection and more pain.

I also encourage you to see a doctor if the dark spot covers more than a quarter of the nail or if it doesn't go away after six months. Sometimes a fungal infection will turn a toenail very dark and thicken it, which requires an over-the-counter or prescription antifungal medication. In super-rare cases, a black spot beneath a nail may be a skin cancer, so it's best to let your doctor assess the discoloration.

Health’s medical editor, Roshini Rajapaksa, MD, is assistant professor of medicine at the NYU School of Medicine.