Black Toenails From Running: Causes, Prevention and What to Do

Black toenails from running are usually a bruise under the nail, caused by your toes repeatedly hitting the front or top of your shoe over many miles. You can usually prevent them with better shoe fit, smarter lacing and short nails, and if you already have one, the best approach is normally to protect it and let it grow out, getting it checked if it is painful, swollen, looks infected or appeared without any injury. Here is what causes it, how to stop it happening again, and when to get help.

What causes black toenails from running?

Every time your foot lands, it moves slightly forward inside the shoe. If the tip of a toe meets the end or the top of the shoe on each stride, those small knocks add up over thousands of steps. The tissue under the nail gets bruised, blood collects beneath it, and the nail turns dark red, purple or black. The medical name for blood under a nail is a subungual haematoma, but most runners just call it runner's toe.

The usual triggers are:

• Shoes that are too small or too short. Not enough length or depth in the toe box means toes hit the end on every stride.
• Shoes that are too loose through the middle. A sloppy fit lets the whole foot slide forward, especially downhill.
• Long runs. Feet tend to swell over a long run, so a shoe that feels fine at mile three can feel tight by mile eighteen.
• Downhills. Running downhill pushes your toes towards the front of the shoe with every step.
• Long toenails. A nail that sticks out past the end of the toe takes the hit first.
• Your foot shape. If your second toe is longer than your big toe, it is often the one that goes black.

Not every dark nail is a running bruise, though. Fungal nail infections can change a nail's colour, and a dark mark that appears without any knock needs checking (more on that below).

Why do black toenails show up in marathon season?

Autumn is when many runners hit their longest runs and their races. More time on feet means more strides, more swelling and more chances for a toe to tap the end of the shoe. Wet weather adds to it: soaked socks and shoes can stretch and loosen, letting the foot move more. Hilly courses and trail races bring long descents. And race day itself is a classic moment, especially if you switch to a lighter, snugger racing shoe you have not run far in.

If a nail turns dark after a long run, treat it as a clue rather than a badge of honour: the fit, lacing or terrain is worth changing before your next big session.

How should running shoes fit to protect your toenails?

Fit is the foundation. Stand up in the shoe with your weight spread evenly and check for roughly a thumb's width of space between your longest toe and the front of the shoe. Remember your longest toe might be your second toe. Then check the toe box is wide and deep enough that your toes are not squashed together or pressing on the top.

• Try shoes on later in the day, when your feet are at their largest, and wear your usual running socks.
• Many runners find their running shoes need to be around half a size bigger than their everyday shoes. Go by fit, not the number on the box.
• Do not overshoot. A shoe that is too big lets your foot slide, which can cause the same problem.
• Build up gradually in new shoes before trusting them on a long run or race.

If you are buying a new pair, our guide on how to choose running shoes walks through fit, cushioning and what to try on in the shop.

Which lacing tricks stop your foot sliding forward?

Even a well-sized shoe can let your foot creep forward if the heel is not held in place. The most popular fix is heel lock lacing, sometimes called a runner's loop. Most running shoes have a spare eyelet at the top for exactly this.

• Lace as normal until you reach the second-to-last eyelet.
• Take each lace straight up into the top eyelet on the same side, leaving a small loop.
• Cross the laces over and thread each one through the loop on the opposite side.
• Pull down and back to snug the heel, then tie as normal.

Keep the lacing over your toes and forefoot comfortable rather than tight, and save the firm hold for the top. If heel lock lacing causes pressure or pain on the top of your foot, loosen it or go back to your usual method. Tighter is not always better.

What else helps prevent black toenails?

Once fit and lacing are sorted, a few habits help.

• Keep nails short. Trim straight across, level with the end of the toe, and smooth any sharp edges with a file. Cutting too short or into the corners can lead to ingrown toenails, so leave a little white.
• Wear smooth, well-fitting socks. Moisture-wicking socks that sit flat without bunching help. Very thick socks can take up toe room, so if you switch thickness, re-check the fit.
• Shorten your stride downhill. Quick, short steps with your weight over your feet reduce how hard you brake and how far your toes slide. Our downhill running technique guide covers this in more detail.
• Build distance gradually. Jumping from a 10-mile long run to 18 miles gives your feet little time to adapt.
• Replace worn shoes. Shoes that have lost their shape can let your foot move around more.

Many of these fixes overlap with stopping rubbing elsewhere on your feet. If you also get sore spots on your heels or toes, our guide on how to prevent blisters when running goes deeper on socks, balm and tape.

What should you do if you already have a black toenail?

If the toe is not very painful and there are no signs of infection, the usual approach is to leave it alone and let the dark area grow out with the nail. That can take many months, and if the nail comes away, a big toenail can take up to around 18 months to grow back fully. Some general tips while you wait:

• Do not drain it yourself. Pricking or heating a nail at home risks infection. If pressure is building, let a GP or podiatrist decide what to do.
• Do not pull the nail off. If it starts to lift, cover it with a plaster so it does not catch on socks, and let it come away in its own time.
• Keep it clean and dry. Wash and dry your feet well after runs.
• Trim carefully. If a loose edge is catching, trim just that edge with clean clippers, rather than cutting into the nail bed.
• Ask a pharmacist. They can suggest dressings, cushioning and pain relief that suit you.

When should you see a GP, podiatrist or pharmacist?

Most runners can manage a black toenail without fuss, but get it checked if:

• The pain is throbbing or getting worse rather than easing.
• The toe is swollen, hot or red, or there is pus, a bad smell or red streaks. These can be signs of infection.
• A dark mark appeared and you cannot link it to any knock or long run. The NHS says to see a GP if you have a dark area under a nail that has not been caused by an injury.
• A dark streak does not move forward as the nail grows, or the colour spreads onto the skin around the nail.
• You have diabetes, poor circulation or reduced feeling in your feet. Get any foot injury checked promptly.
• Your nails keep going black despite fixing fit and lacing. A podiatrist can look at your foot shape, gait and shoes.

A pharmacist is a good first stop for minor concerns. If something feels very wrong, do not wait it out.

Can you keep training with a black toenail?

Often, yes, if it is not painful. Many runners carry on with easy runs while the nail grows out, keeping it covered and the shoes laced well. If running hurts, or the nail is lifting, give it a few days off pounding and keep your fitness ticking over another way: cycling, swimming once any open skin has healed, strength work or a seated upper-body session. Pain is a signal to ease off, not push through.

This is where a plan that mixes running, strength, HIIT and mobility is useful. With Edge Pro, your plan already blends all four, and you can message a real coach anytime to swap a run for a bike or strength session while a sore toe settles, then build your running back up.

If you have an autumn race coming up, test your race shoes, socks and lacing on at least one long run beforehand, trim your nails a few days before (not the night before), and keep a plaster in your race bag.

This article is general guidance, not medical advice. If you are new to exercise, returning from injury, pregnant, or managing a health condition, speak to your GP before starting, and see a physio for any niggle that does not settle.