A black toenail is caused by a blood blister underneath the nail. The collection of blood under the nail discolors it and can cause pressure and pain, but black toenails generally are not painful. In most cases, the toenail eventually falls off.
A black toenail occurs when your toe becomes bruised from bumping against the end of your running shoe. This can happen if you do a lot of downhill running or racing or if your shoes are too small. Usually, runners with a Morton's foot type (the second toe is longer than the first toe) are most susceptible to having bruised second toenails.
You need to drain the blood to remove the pressure. To do this, swab your toenail with alcohol. Then take a paper clip or other sharp, narrow object, heat it in a flame and push it through the toenail. Drain the blood, apply an antiseptic and cover the hole with an adhesive bandage.
The best way to prevent black toenails is to wear shoes that fit properly. The toe box should be wide enough and the length of the shoe long enough so your toes don't bump against the shoe. You should have about a half-inch of space between the end of your longest toe (not necessarily the big toe) and the top of your shoe. Blister-free socks may help prevent friction.
Can you run with it?
Usually. If the toe throbs with pain, it's best to take a couple of days off and let the toenail heal.
Run The Planet thanks Running The World for the permission to reprint the article “Fourteen common foot ailments and their cures”. Text copyright © 2001 by Running The World.