Black toenails can ruin races and hamper your running – here’s how to treat and prevent them

Black Toenails - treatment and prevention

Black toenails are a particularly common injury for first-time marathon runners (Illustration: Peter Greenwood)

For some triathletes losing a toenail during a race – especially Ironman – is normal. But before the nail comes off completely, it usually turns black.

The black toenail is caused by fluid and blood from broken capillaries building up under the nail after it has undergone the repetitive trauma of hitting the end of the running shoe, the front of the shoe folding against the nail or the impact of footfalls damaging the toe’s tissue from underneath.

The nail will not turn immediately black, but if there’s a feeling of painful pressure the day after your race, it’s likely that the toenail will eventually turn that way.

A contributing factor to black toenails is how used your body is to extra blood being thrown into the toes each time the foot is swung forward during running – the more you’re running beyond your distance limit, the more likely black toenails are.

Black toenails – symptoms

  • Pain in toe while running
  • Pain and feeling of pressure increasing following running
  • Discolouration and darkening of nail compared to healthy nails


  1. Make sure your trainers fit properly. You need to make sure they’re not so big that your feet slide against the ends inside them, nor so small that there’s rubbing or pressure from too tight a fit.
  2. Trimming and filing your toenails before you run can help stop the nail hitting the inside of the shoe and avoid bleeding underneath. It can also reduce the pressure of sharp edges digging in.
  3. Avoid sudden training regime changes that require sudden big mileage increases – build up in small increments to allow your feet’s tolerance to adjust naturally to longer runs.
  4. If you do suffer a black toenail, the pain may well subside after about 24 hours. If it becomes intolerable to walk on, you may need to visit your GP, where a hot needle cautery is used to punch a hole in the back of the nail to let the blood and fluid out.
  5. You can relieve the pressure of black toenails at home – it doesn’t hurt if done right, but if in doubt, don’t attempt it. Sterilise a paperclip by putting it in boiling water. Unfold one end of the paperclip and holding the other with a glove to avoid heat build up, hold the tip under a lighter until it turns orange. Place it lightly on the affected toenail and the tip should melt the nail away to leave a hole for the blood and fluid to escape. Do this gently and there isn’t any pain when perforating the nail.

Find out more about common triathlon injuries or read more about how to avoid triathlon injuries on TriRadar.