Black toenails are almost always caused by bleeding trapped under the nail, a condition called a subungual hematoma. An injury or repeated pressure damages tiny blood vessels in the nail bed, and because the nail plate sits flush against the tissue beneath it, the leaked blood has nowhere to go. It pools, creates pressure, and turns the nail dark red, purple, or black. Less commonly, black toenails result from fungal infections, poor circulation, or, rarely, melanoma.
Trauma and Repetitive Pressure
The most frequent cause is straightforward physical injury. Stubbing your toe hard, dropping something heavy on it, or crushing it in a door can rupture blood vessels in the nail bed almost instantly. The trapped blood discolors the nail within hours, and the throbbing pain comes from pressure building in a space with no room to expand.
Not all trauma is sudden, though. Repetitive, low-grade pressure does the same thing over time. Runners are especially prone to this. Every stride pushes the toes forward, and the nail of the big toe repeatedly slams into the front or side of the shoe. Running downhill makes it worse by forcing the foot even farther forward. Over hundreds or thousands of steps, this microtrauma produces the same blood pooling as a single blunt hit. The result, sometimes called “runner’s toe,” is a gradually darkening nail that may not hurt much day to day but eventually turns fully black.
Shoes that are too tight, too short, or too loose (allowing the foot to slide) all increase the risk. Hikers, soccer players, and anyone spending long hours on their feet in poorly fitting footwear can develop the same problem.
What Happens After a Bruised Nail
Small hematomas covering less than about 25% of the nail that aren’t painful will reabsorb on their own. Your body gradually breaks down the trapped blood, and the discoloration grows out as the nail replaces itself. If the hematoma covers more than 25% of the nail or is causing significant pain, a doctor can relieve the pressure by making a small hole in the nail plate to drain the blood. This is a quick, in-office procedure that typically brings immediate relief.
Toenails grow slowly. After trauma, expect 6 months to 2 years for a toenail to fully replace itself, with an average around 18 months. If the nail falls off entirely, the regrowth timeline is similar. During that time, the dark color gradually moves toward the tip and eventually gets trimmed away.
Fungal Infections
Nail fungus doesn’t always look like the stereotypical yellow, crumbly nail. In advanced cases, the nail can thicken, turn opaque, and become yellow-brown to nearly black. The fungus typically enters through the skin around the nail or underneath the free edge at the tip, then spreads into the nail bed. The resulting inflammation and debris buildup beneath the nail plate cause the color change.
A few clues distinguish fungal discoloration from a bruise. Fungal nails tend to thicken and crumble at the edges. The discoloration usually starts at one end of the nail and spreads, rather than appearing as a sudden dark patch. There’s typically no memory of a specific injury. Multiple nails may be affected, and athlete’s foot on the surrounding skin is common. Treating nail fungus takes patience: even after successful antifungal treatment, it can take 12 to 18 months for the infected nail to fully grow out and be replaced by healthy nail.
Poor Circulation
Reduced blood flow to the feet can cause nail changes that range from slow growth and brittleness to dark discoloration. Peripheral artery disease, in which narrowed arteries limit blood reaching the extremities, is one of the more common circulatory causes. Risk factors include diabetes, high blood pressure, high cholesterol, smoking, and older age. You might also notice a pale or bluish tint to the skin on the feet, decreased hair growth on the legs, or wounds that heal slowly. In these cases, the black toenail is a symptom of a bigger vascular problem, not a standalone condition.
Chemotherapy and Medications
Certain cancer treatments can cause increased pigmentation in the nail plate, producing dark brown or black streaks or patches. Unlike bruise-related discoloration, this pigmentation does not always go away after treatment ends. It tends to affect multiple nails at once and develops gradually over the course of chemotherapy cycles. If you’re undergoing treatment and notice nail color changes, your oncology team can confirm whether the discoloration is medication-related.
When Black Means Melanoma
Subungual melanoma, a type of skin cancer that develops under the nail, is rare but serious. It typically appears as a dark streak or band running lengthwise through the nail rather than as a diffuse bruise-like patch. Dermatologists evaluate these streaks using a set of warning signs organized by the letters A through F:
- Age and ancestry: More common in adults over 50 and in people with darker skin tones.
- Band characteristics: A brown or black band wider than 3 millimeters, with irregular or blurred borders.
- Change: The band is growing wider, darker, or changing shape over time.
- Digit: The big toe (or thumb) is most commonly affected.
- Extension: Pigment spreading from the nail onto the surrounding skin of the cuticle or nail fold, known as Hutchinson’s sign.
- Family history: A personal or family history of melanoma or unusual moles.
The key distinction from a bruise is behavior over time. A bruise grows out with the nail. A melanoma streak stays in place or widens because the pigment-producing cells at the base of the nail keep generating it. Any dark spot or streak that doesn’t move toward the tip of the nail as it grows, or that spreads onto the skin around the nail, needs a biopsy to rule out melanoma. The diagnosis is confirmed only through tissue examination, not visual inspection alone.
How to Tell What’s Causing Yours
A few practical questions can help narrow it down. If you remember a specific injury, or you’ve recently increased your running mileage or worn tight shoes, trauma is the most likely explanation. Watch for the discoloration to slowly migrate toward the tip of the nail over the following months. If it does, that’s confirmation it was a one-time bleed.
If the nail is thickened, crumbly, or lifting away from the nail bed with no history of injury, fungal infection moves to the top of the list. If multiple toenails are affected or you have risk factors like diabetes or poor circulation, those causes deserve consideration. And if a dark streak appeared without trauma, isn’t growing out, or is getting wider, that warrants a dermatology visit and likely a biopsy.
Black toenails that persist for more than a few months without any sign of growing out, or dark spots that seem to be spreading into the surrounding skin, should not be dismissed as a simple bruise.