Why Runners Lose Toenails (And How to Prevent It)

Black toenails — and sometimes even losing a toenail entirely — are among the most common problems runners experience, especially during long races and heavy training blocks. While they’re usually harmless, they can be painful and sometimes signal that something in your running setup needs adjusting.

Most runners first notice their toenail turning dark after a race or long run, and many wonder whether it’s something to worry about. In some cases the nail eventually falls off altogether.

Here’s why runners get black toenails — and what you can do to prevent them.

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Why Runners Get Black Toenails

Black toenails happen when repeated trauma damages the nail bed, causing blood to pool underneath. For runners, this usually comes down to:

• Repetitive impact: toes hitting the front or top of your shoe over and over.
• Downhill running: your feet slide forward, slamming into the shoe.
• Poorly fitting shoes: too tight, too loose, or the wrong shape.
• Swelling during long runs: feet naturally expand due to increased blood flow and fluid retention, making shoes feel tighter by the mile.

Runner’s toe isn’t usually an issue on short runs, but during longer efforts — like marathons, ultras, or even your weekend long run — the repetitive impact adds up. Each micro-trauma may feel minor in the moment, so you might not notice anything until you take your shoes off and spot the bruising, or even a few days later once it fully develops.

How to Prevent It

You can’t always avoid runner’s toe, but these steps make a big difference in keeping your toenails intact:

Choose the right shoe fit
Even if you’ve worn the same size for years, long runs and races often require a half to full size up, or a wider toe box. This extra space lets your feet swell naturally and prevents pressure on your nails.

Keep your toenails short and straight
Trim them straight across (not rounded and not too short) to reduce pressure points and lower the risk of fungal or dystrophic nails.

Wear good socks
Avoid thick, generic socks that add pressure. Instead, choose running-specific socks: thinner in the toes, cushioned in the sole, and made from moisture-wicking fabrics. These help cut down on friction, hot spots, and blisters.

Lace up smart
Snug lacing keeps your feet from sliding forward. Experiment with different lacing techniques to find what works best for your foot shape.

Lube or tape your toes
Just like with blisters, lubricating or taping your toes reduces friction and impact. Products like Squirrel’s Nut Butter or Body Glide Foot Balm work well.

Build mileage gradually
Your feet, like the rest of your body, need time to adapt. Stick to the 10% rule when increasing weekly mileage to avoid overloading them.

Consider extra protection
If black toenails are a recurring problem, try toe protectors or cushioned sleeves to reduce friction and absorb the shock between your toes and the shoe.

When to See a Doctor

Most black toenails are harmless and will eventually fall off and regrow on their own. But there are times when it’s smart to get them checked out:

• Severe pain or pressure: sometimes the blood buildup under the nail (a subungual hematoma) needs to be drained by a doctor to relieve pain.
• Signs of infection: redness, swelling, warmth, pus, or increasing pain are warning signs that need medical attention.
• Repeated injuries to the same nail: constant trauma can cause permanent changes to the nail or lead to chronic problems.
• If the nail looks unusual: dark streaks or discoloration that don’t improve may need to be evaluated to rule out other conditions.

If you’re ever unsure, it’s best to play it safe and check in with a podiatrist.

FAQs About Runner’s Toe

Do black toenails mean my shoes are too small?
Not always. Poorly fitting shoes are a common cause, but it’s not just about size. If your toes repeatedly hit the front of the shoe, you may need to size up or look for a wider toe box or a different shoe shape.

How long does it take for a black toenail to heal?
It usually takes several months. The damaged nail will either grow out or fall off, and a new one will gradually replace it.

Should I remove a black toenail?
No. It’s best to let it fall off naturally. Forcing it off increases the risk of infection and slows healing.

Can black toenails be permanent?
Most of the time, the nail grows back normally. However, repeated trauma — especially from marathons or ultramarathons — can sometimes cause nails to become thickened or slightly misshapen over time.

The Bottom Line

Black toenails may feel like a rite of passage for runners, but they don’t have to be. With the right shoes, smart foot care, and a few simple prevention strategies, you can avoid runner’s toe and keep your focus where it belongs — on the miles ahead.