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About 27 percent of novice runners experience an injury each year, and this number jumps up to 52 percent for marathon runners.

If adjusted for total hours run, novice runners are about twice as likely to have an injury as people who run regularly.

One usually minor injury that many runners experience is runner’s toenail. Runner’s toenail is when one of your toenails turns black because of the repeated stress of running and the damage it can cause to blood vessels that feed the toenail.

Although it’s often painless, and some runners see it as a badge of honor, it can lead to several potentially painful complications, such as:

Keep reading to find out why runners often get runner’s toenail, how you can treat it, and how you can keep it from coming back.

Runner’s toenail, also known as jogger’s toenail, is when your nail or surrounding area becomes black from the repeated stress of running. The discoloration comes from blood that leaks from broken blood vessels.

Medically, runner’s toenail is known as subungual hematoma. Subungual means under the nail. Hematoma means a collection of blood outside a blood vessel.

Repetitive stress usually causes runner’s toenail.

Each time your foot touches the ground when you’re running, the tips of your toes slam into the front of your shoe.

The inside of your shoe is relatively soft, so this impact only causes a microscopic amount of damage known as microtrauma.

The concept of runner’s toenail is similar to the idea of lightly hitting your toenail thousands of times per week in the same spot with a toy hammer.

Each time you hit it, you’re unlikely to do damage to the nail. However, thousands of microtraumas can damage your blood vessels and cause them to leak blood. The more times you hit your toe, the more damage accumulates.

You’re most likely to develop runner’s toenail in your big toe or second toe, since they extend the farthest in your shoe.

Other causes of black toenails

Some runners develop black toenails after they contract a fungal infection. Runners are particularly susceptible to infections since fungi thrive in moist environments, like sweaty socks.

Onychomycosis is a fungal infection of your nail that can cause discoloration. It usually causes a yellow-brown discoloration, but it may also lead to darkness under your nail from debris buildup.

Other less likely reasons you may have a black toenail include:

However, if you notice your black toenail after increasing your mileage, it’s much more likely that your training has caused your black toenail.

Repeated stress to your toe causes runner’s toenail. So, minimizing stress to this area may help you prevent it. Here are some preventive steps to take:

  • Buy properly fitting shoes. It’s a good idea to buy shoes loose enough to give your toes room to move but not so loose that your foot slides in the shoe. Buying your shoes from a store that specializes in sportswear is a good way to get a professional fitting.
  • Keep your toenails short. Keeping your toenails short can help you minimize the amount the end of your nail hits the front of your shoe.
  • Work on your technique. Novice runners are more likely to develop running injuries than regular runners per the same number of miles. Working on technique may help reduce your injury risk.
  • Tie your shoes. Keeping your shoes tight can minimize the amount your feet slide while running.
  • Slowly increase mileage. Many coaches recommend only increasing your mileage by 10 percent at a time. One study found that novice runners who increased their mileage by more than 30 percent over 2 weeks were more vulnerable to injury than runners who increased mileage by less than 10 percent.
  • Wear cushioning socks. Socks that provide cushioning may be able to absorb some of the force from running. However, they shouldn’t fit so snug that your foot rubs against the end of your shoe.

Although there hasn’t been any research specifically looking at risk factors for runner’s toenail, a 2015 review of studies examined potential risk factors for running injuries in general. It found strong or moderate evidence that:

  • Men were more likely to experience running injuries than women.
  • A history of previous injury was linked to future injury risk.
  • Wearing orthotics or inserts was linked to an increased risk of injury.

Researchers also found that the following were potential risk factors for women:

  • age
  • previous sports activity
  • running on concrete surfaces
  • running between 30 and 39 miles per week
  • wearing running shoes for 4 to 6 months

For men, potential risk factors were:

  • running more than 40 miles per week
  • running between 20 and 29 miles per week
  • having less than 2 years of running experience
  • restarting running after a break

Some of the most common symptoms of runner’s toenail include:

  • black or dark red discoloration
  • pain
  • loose toenails
  • blood blisters
  • pressure under your nail
  • loss of toenail

If you develop an infection, you may experience:

  • sharp pain
  • redness
  • swelling
  • fever
  • oozing pus
  • bad smell

If you aren’t experiencing pain or complications, no specific treatment is needed for runner’s toenail.

In some cases, enough blood can collect under the toenail that the nail begins to lift from its bed. Often, this causes a sharp or throbbing pain.

If you’re experiencing this type of pain, it’s a good idea to visit your doctor. Your doctor can use a heated needle to make a small hole in your nail to drain the blood.

If your nail falls off, clean it with an antibiotic cream, and cover the toe with a bandage before visiting your doctor. Your doctor can advise you if any other treatment is needed for the nail to grow back properly.

Also visit your doctor if you notice signs of an infection, such as pus or swelling. Your doctor may give you a prescription for oral antibiotics.

Many people never develop any long-term complications from runner’s toenail.

Runner’s toenail usually heals when you:

  • reduce your mileage
  • change your running shoes
  • fix running mechanics that may be putting more stress on your toes

Working with a running coach is a great way to identify and fix biomechanical issues that may be leading to injuries.

If your toenail falls off, it can take more than a year for it to grow back. The area may remain tender for a while afterward, but you can return to running as soon as the pain subsides.

Runner’s toenail is when you develop black discoloration beneath the nails of one of your toes from the repetitive stress of running.

Runner’s toenail often doesn’t cause any symptoms other than discoloration. It often goes away by itself when you reduce your training load or switch shoes. In some cases, it may lead to the loss of your toenail.

If you notice signs of infection, such as pus or swelling, or if your nail is lifting from the nail bed, it’s a good idea to visit your doctor. They can recommend the best treatment options for you.