Black toenails from running are usually subungual hematomas, blood trapped under the nail plate after the toe repeatedly strikes the front or top of the shoe. They develop from acute injury or chronic repetitive impact, and prevention centers on shoe fit, nail care, and friction control.
A bruised nail often grows out on its own. It may thicken, loosen, lift, and fall off over time. Runners should protect the nail and watch for pain, infection, or changes that need care.
Medical information disclaimer: This article is for general educational purposes only and does not provide medical advice, diagnosis, or treatment. Always consult a physician or other qualified health professional about symptoms, medications, tests, or treatment decisions.
Table of Contents
- Why does the nail turn black?
- Who gets runner's toenail most often?
- How can you prevent black toenails?
- What should you do when a nail turns black?
Why does the nail turn black?
A subungual hematoma means blood is trapped beneath the hard nail plate. Michigan Foot Doctors explains that the toe repeatedly hits the shoe during running, injuring the small vessels under the nail, in its runner's toenail guide. The pooled blood then looks dark blue, brown, or black through the nail.
Sports medicine physician Dr. Jordan Metzl says both a single hard stub and repeated small impacts can cause this bruising in runners. Downhill steps, sudden stops, and long runs increase the number of toe-to-shoe contacts. A nail can also bruise when the toe presses against the top of a low or tight toe box.
Who gets runner's toenail most often?
Distance runners, trail and downhill runners, and hikers face the highest risk. Feet swell during long or hot efforts, and the foot slides forward inside the shoe on descents. Higher mileage means more collisions between toe and shoe.
Heat, long descents, and loose lacing make sliding worse. The second toe is often affected when it is longer than the big toe. Tight, short, or narrow shoes raise risk because the toes have no room to spread and move.
How can you prevent black toenails?
Start with shoe fit. Baylor College of Medicine advises buying running shoes about one-half size larger, with roughly one-half inch between the longest toe and the shoe end, in its runner's toe interview. Choose a wide toe box and use secure lacing to keep the foot from sliding forward.
Keep the other friction points low as well: Marathon Handbook summarizes similar protection steps for cushioned socks, short nails, and toe padding in its bruised-toenail prevention guide. Replace worn shoes before the upper stretches or the sole wears unevenly. Re-check fit in hot weather, when swelling is more likely.
- Trim toenails short and straight across so the nail edge does not strike the shoe.
- Wear moisture-wicking, seamless, cushioned socks to reduce movement and dampness.
- Add silicone toe pads or moleskin where one toe rubs or presses.
What should you do when a nail turns black?
Leave a bruised but fairly comfortable nail alone and let it grow out. Cleveland Clinic podiatrist Dr. Georgeanne Botek notes that the nail may thicken, loosen, lift, and eventually fall off.
Do not pull a lifting nail because it can tear the nail bed. Do not puncture or drain a bloody nail at home. Michigan Foot Doctors recommends rest, ice, elevation, temporarily lower mileage, and medical care for severe throbbing pain, discoloration covering more than half the nail, deformity, or infection signs in its subungual hematoma guide. Seek care for these warning signs:.
- Severe or worsening throbbing pain
- Rapidly expanding dark area or injury with deformity
- Redness, swelling, warmth, pus, red streaks, or fever
- A dark mark that does not grow out, widens, has irregular borders, spreads beyond the nail, or appears without injury, which rarely can be fungal melanonychia or subungual melanoma and needs prompt dermatology evaluation