What Causes Black Toenails After Running?
May 29, 2026
Few things are more alarming for runners than taking off their shoes after a workout and discovering a toenail has turned black. While it can look serious, black toenails are surprisingly common among runners, hikers, and athletes who spend long hours on their feet.
In many cases, a black toenail is the result of repeated trauma inside the shoe rather than a serious medical condition. However, not every dark toenail is caused by running, and knowing the difference can help you determine when to seek professional care.
Let's explore why black toenails develop, how they're treated, and what you can do to keep them from happening again.
A black toenail typically occurs when blood collects beneath the nail plate after repeated trauma or pressure. This condition is known as a subungual hematoma.
The discoloration may appear:
The affected nail may also become painful, loose, or thickened depending on the severity of the injury.
The most common cause is repetitive impact between the toe and the inside of the shoe.
Every time your foot strikes the ground, your toes move slightly within your shoe. Over the course of thousands of steps, this repeated motion can create enough trauma to damage small blood vessels beneath the nail.
One of the leading causes of black toenails is wearing running shoes that do not provide adequate room in the toe box.
When shoes are too short or too narrow:
Even shoes that feel comfortable while standing may become problematic during longer runs when feet naturally swell.
Many runners notice black toenails after races that include significant downhill sections.
Running downhill causes the foot to slide forward inside the shoe, increasing pressure on the toes. This repeated impact can quickly lead to bruising beneath the nail.
Trail runners and hikers are particularly susceptible to this problem.
The longer you run, the greater the cumulative stress on your toes.
Marathon runners, ultramarathon participants, and endurance athletes frequently develop black toenails because of the sheer number of foot strikes involved during training and competition.
Toenails that are too long are more likely to strike the inside of the shoe.
Long nails can:
Keeping toenails properly trimmed may reduce the risk of injury.
Certain foot types are naturally more prone to black toenails.
Factors that can increase risk include:
These structural differences may increase pressure on specific toes during running.
Symptoms vary depending on the severity of the injury.
Common signs include:
Some runners experience significant pain initially, while others notice the discoloration without any discomfort.
Sometimes.
If the injury is significant, the damaged nail may gradually separate from the nail bed and eventually fall off.
While this can be alarming, a new nail often begins growing underneath.
However, regrowth can take time:
The new nail may initially appear ridged or irregular before normalizing.
Not all black toenails are caused by trauma.
A dark nail should be evaluated by a podiatrist if:
In rare cases, a dark streak or spot beneath the nail may represent a more serious condition requiring prompt medical evaluation.
Treatment depends on the severity of the injury.
Many cases simply require:
The discoloration often grows out with the nail over time.
If significant blood has accumulated beneath the nail, a podiatrist may perform a procedure to relieve pressure and reduce pain.
This treatment can provide immediate relief when performed soon after the injury occurs.
If the nail becomes detached or partially detached, professional care may help prevent infection and promote healthy regrowth.
Prevention often comes down to proper footwear and training habits.
Running shoes should generally provide about a thumb's width of space between your longest toe and the front of the shoe.
Your feet swell during activity, so extra room is important.
Quality running socks help reduce friction and excessive foot movement inside the shoe.
Trim nails straight across and avoid allowing them to become excessively long.
Old shoes lose support and cushioning, increasing foot movement and impact forces.
If abnormal foot mechanics contribute to repeated toenail injuries, custom orthotics may improve foot function and reduce pressure points.
Schedule an evaluation if:
Early treatment can help prevent complications and identify underlying issues contributing to recurring injuries.
Black toenails are a common problem among runners, usually caused by repeated trauma from shoes, long-distance running, or downhill activity. While most cases are harmless and resolve over time, persistent pain, recurring injuries, or unexplained discoloration should never be ignored.
Proper footwear, good running habits, and early podiatric care can help keep your feet healthy and prevent black toenails from interfering with your training goals. If you're dealing with a painful or recurring black toenail, a podiatrist can help identify the cause and recommend the most effective treatment. Contact our office today with Dr. Gireesh Reddy or Dr. Sirisha Pokala by calling (469) 573-3427 or by booking an appointment online.