I have two blackened toenails from running a marathon.

"That blackness in the toenails comes from bleeding under the nail, called a subungual hematoma," explains Kirk Lebensfeld, D.P.M., a New York City podiatrist. It generally results from chronic pounding on the pavement or some other repetitive trauma. Ill-fitting, too-tight running shoes or having one toe that is longer than the others (and therefore, subject to more friction) also can cause blackening bruising. To alleviate the pressure under the nail, a podiatrist will drill a tiny hole through it and express the blood. "You might have some residual blackness, and you could temporarily lose the nail, but it should eventually clear up," says Dr. Lebensfeld. In the meantime, wear thicker socks, use running shoes with a roomier toe box, and keep marathon runs to a minimum.