Black toenails are fairly common among runners, especially those training for long-distance races. First, the toenail appears blackened and the nail will usually fall off when a new nail eventually grows in. Runners who are training for a marathon or do a lot of downhill running
are the most likely candidates for black toenails because their toes are constantly rubbing up against the front of their shoes. You're also more likely to get black toenails if you run in warmer weather because your feet swell more when it's hot.
Black toenails are caused by constant rubbing of your toe against the front of your shoe. A blood blister forms under the nail, and the blister can't breathe, so it takes a lot longer to heal.
To prevent black toenails, make sure that you're wearing the correct running shoe size
(at least 1/2 size bigger than your street size; you should have plenty of room in the toebox). Trim your toenails regularly, and keep your foot dry for as long as possible during your long runs. Be sure to wear good wicking socks
, not cotton ones. Lace your shoes tighter along the front if you're doing a lot of downhill running.
Once you have a black toenail, it's best to leave it alone, as long as the pain is manageable. The pain is usually the worst on the first day and then lessens each day after. The damaged part of the nail is gradually pushed off, and a new nail will replace it. Don't force the old nail off -- it will fall off on its own. If at any point you notice redness and infection, see a doctor.