How to Prevent and Treat Black Toenails From Running

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If you are training for a long-distance running race, you may be familiar with getting a black toenail. This condition is a bruise or blood blister under the toenail. Some runners consider black toenails to be a rite of passage, but they aren't inevitable and it's possible to avoid getting them. They can be painful and you need to know the safe way to treat them.


First, the toenail appears blackened and may be painful. The black color is due to blood under the nail. If there is enough blood under the nail it may be raised or swollen, which is likely to lead to pain. If the nail is traumatized enough it will fall off. Often, it remains attached until the new nail eventually grows in and pushes the old nail out. However, sometimes the nail will fall off earlier, leaving a smooth nail bed until the new nail grows over it.

Causes and Risk Factors

Black toenails are caused by constant hitting or rubbing your toe against the front of your shoe. A blood blister forms under the nail. Since the blister can't breathe as it might on uncovered skin, it takes a lot longer to heal.

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 long distances in warmer weather because your feet swell more when it's hot.

Prevention of Black Toenails

To prevent black toenails, make sure that you're wearing the correct running shoe size (at least a half size bigger than your street size; you should have plenty of room in the toebox). You should have one thumbnail's distance between your longest toe and the end of the shoe. Buy new running shoes at the end of the day when your feet are most swollen. Trim your toenails regularly. Sometimes an untrimmed toenail can hit the front of your shoe and lead to bleeding beneath the nail. Forming blisters on your toes risks that they will be large enough to extend under the nail. You can reduce your risk of toe blisters by keep your foot dry for as long as possible during your long runs. Be sure to wear good wicking socks, not cotton ones.

You can help prevent your foot sliding forward in your shoe by lacing your shoes tighter at the ankle. Use that last set of eyelets to form a lace lock. This is especially important if you're doing a lot of downhill running.

Although black toenails commonly occur when training in warm weather (when your feet swell), you should also be careful when running in cold weather. If you're wearing thick socks or two pairs of socks to stay warm, your running shoes may end up being too tight, putting you more at risk for black toenails.

Treatment of Toenails

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. If it's extremely painful, a doctor can put a hole in the nail to relieve the pressure.

The damaged part of the nail is gradually pushed off, and a new nail will replace it. Don't try to force the old nail off—it will eventually get loose and fall off on its own once the new nail grows in. If you're worried about how your toenails will look, it's OK to put nail polish on the black toenail and on the new nail coming in.

If at any point you notice redness and infection or the pain gets worse, get it checked out by a healthcare professional.

View Article Sources
  • Subungual Hematoma, American Osteopathic College of Dermatology.
  • Vonhof J. Fixing Your Feet: Prevention and Treatments for Athletes Wilderness Press, 2011.