Black Toenail: What Is It and How Do I Prevent It?

The so-called “black toenail” is all too familiar to runners. Most athletes consider black toenails as a harmless phenomenon, a sort of “badge of honor” associated with being active. However, the presence of black toenails can provide important information for runners and should never be ignored or dismissed.

What Is a Runner’s Black Toenail?

Toenails turn black when blood collects and settles under the nail because of repeated trauma from friction, rubbing, or contact. Most commonly, running-related black toenails appear slowly over time, but can also develop suddenly after a race or training run.

Why Do Runners Get Black Toenails?

The standard runner’s black toenail is typically caused by trauma from the nail  constantly hitting the front of a running shoe.

Am I Getting Black Toenail(s) Because My Shoe Is Too Big, Or Because My Shoe Is Too Small?

The answer is YES!  Improper shoe fit is the main risk factor for the black toenail:

  • ]A shoe that is too small puts the toenail right up against the front of the shoe, causing trauma with every step, especially for forefoot strikers.
  • A shoe that is too large will allow the foot to slip forward, constantly hitting the front of the shoe.
  • A shoe that is the proper length can still cause black nails if your shoe is too wide for your foot. Several shoe companies make running shoes in varying widths.
  • Socks can affect shoe fit, especially in weather extremes that make the sock slippery.

 Are There Other Run-Related Causes of Black Toenails?

Even in well-fitting shoes, an intense training regimen filled with speed work, races, or long runs can lead to black toenails. Uneven terrain or big changes in elevation/descent can increase trauma to the front of the foot. Also, wet conditions can cause foot slippage or a change in gait that can traumatize toenails. Some people have a “Morton’s second toe” which is longer than the big toe, increasing risk of black toenails even in ideal shoes, conditions, and training.

 What Else Can Make a Toenail Turn Black?

Melanoma, a type of skin cancer, sometimes develops as a growth in the skin under a toenail. Typically, the nail will have a dark spot that continues to expand over time, growing past the nail margins and/or pushing the nail up. Melanomas need immediate, urgent evaluation. A toenail can also become discolored from fungal infections in the skin under and around the nail. Toes can turn black from a lack of circulation causing gangrene, which is death of body tissue. If there is any doubt what is causing your black toenail, see your doctor or podiatrist.

How Do I Treat My Black Toenail?

The standard runner’s black toenail from bleeding that develops over time is a painless, relatively benign problem that does not require any direct action. However, a toenail that turns black suddenly can be very painful because rapid bleeding causes intense pressure under the nail, and a doctor can poke a hole in the nail to relieve pressure if treated within 24 hours. In either case, the most definitive treatment is to find out the cause and correct it. Consider the following causes and make changes as needed:

  • Does your shoe fit properly?
  • Is there a particular workout causing trauma to your toenails?
  • Is terrain a factor?
  • Do you have a medical problem unrelated to running that is causing nail discoloration?

Eshwar Kapur MD, is a sports medicine physician who practices at Kaiser Permanente South San Francisco, where he specializes in running medicine. Dr. Kapur ran varsity cross-country and track and field for the University of Virginia and has been running for more than 25 years without injury. He has participated in medical coverage for races around the country including the New York City Marathon, the Cherry Blossom Run in Washington, D.C., and the Nike Women’s Marathon, where he was featured as a guest speaker at their Expotique pre-race expo.