A black toenail is almost always caused by blood trapped beneath the nail after some kind of injury, whether that’s a single obvious event like stubbing your toe or gradual damage you never noticed. Less commonly, it can signal a fungal infection, a systemic health condition, or, in rare cases, melanoma. The color itself comes from blood or pigment pooling in the tight space between your nail plate and the nail bed underneath, where there’s normally no gap at all.
Trauma Is the Most Common Cause
When blood vessels in the nail bed get injured, they leak blood that has nowhere to go. It pools under the nail plate and puts pressure on it, causing both pain and discoloration. This is called a subungual hematoma. The color typically starts out red or purple and gradually darkens to brown or black over the following days.
You don’t need a dramatic injury for this to happen. Dropping something on your foot, jamming your toe into furniture, or having someone step on it can all do it. The pain is usually immediate and throbbing because of the pressure building in that small, enclosed space.
Repetitive Pressure Without a Single Injury
Sometimes a toenail turns black without any moment you can point to. This is especially common in runners and hikers. Every stride pushes the toenail into the front or side of the shoe, and over time that repeated friction causes bleeding under the nail. It’s known informally as “runner’s toe,” and it most often affects the big toe or the second toe.
The fix is usually about footwear. Getting properly sized at a shoe store, making sure there’s enough room in the toe box, and keeping nails trimmed short all reduce the friction that causes the problem. If your shoes are even slightly too small or you’re running downhill frequently, the cumulative stress adds up faster than you’d expect.
Fungal Infections Can Darken Nails Too
A fungal toenail infection, or onychomycosis, usually turns nails yellow or white, but certain fungi produce brown or black discoloration instead. The key difference from a bruise is that fungal nails also thicken, become brittle or crumbly, and may separate from the nail bed. The changes happen slowly over weeks or months rather than appearing after an injury.
If your black toenail is also thick, chalky, or lifting away from the skin underneath, a fungal infection is more likely than trauma. These infections don’t resolve on their own and typically require antifungal treatment to clear.
Systemic Conditions That Affect Nail Color
In some cases, dark nail pigmentation reflects something happening elsewhere in the body. Chronic kidney disease can increase melanin production, turning the outer portion of the nail bed brown. Hyperthyroidism can cause similar brown discoloration. Excessive fluoride intake has been linked to nails turning brown or black.
These causes are far less common than trauma or fungal infections, and they almost never show up as an isolated symptom. If a systemic condition is responsible, you’d typically notice other changes in your health as well.
It’s also worth knowing that dark longitudinal streaks running the length of the nail are a normal variant in people with darker skin tones. These streaks are caused by benign pigment deposits and don’t indicate disease.
When to Think About Melanoma
Subungual melanoma, a type of skin cancer that develops under the nail, is rare but serious. It accounts for a larger share of melanoma cases in Black, Asian, and Native American populations, where it can represent up to one-third of all melanomas. Peak incidence falls between the ages of 40 and 70.
Dermatologists use an ABCDEF system to identify warning signs:
- Age and ancestry: Most common in the 50 to 70 age range, and in African American, Asian, and Native American individuals
- Band: A brown-to-black band wider than 3 millimeters with irregular or blurred borders
- Change: The band is getting wider, darker, or changing shape over time
- Digit: The thumb and big toe are most commonly affected
- Extension: Pigment spreading beyond the nail onto the surrounding skin fold, known as the Hutchinson sign
- Family history: A personal or family history of melanoma
The biggest red flag is pigment that extends from under the nail onto the skin around it. A bruise from trauma stays contained under the nail plate. Melanoma can bleed beyond those borders. If your dark nail isn’t growing out over time, or if the dark area is expanding rather than staying the same, that warrants a prompt evaluation.
How a Bruised Toenail Heals
Toenails grow at roughly 1.5 millimeters per month, which means a fully black nail can take 6 to 12 months to grow out completely. In some cases, full regrowth takes up to 18 months. You’ll gradually see normal nail appearing at the base while the dark portion moves toward the tip.
Small, relatively painless bruises don’t need any treatment. They’ll resolve on their own as the nail grows. Larger hematomas covering more than half the nail bed, especially if they’re throbbing, may benefit from drainage, a quick procedure where a small hole is made in the nail to release the trapped blood and relieve pressure. This works best within the first day or two, before the blood clots. After that window, drainage won’t be effective and the hematoma simply has to grow out.
If the nail is cracked, split, or partially torn away from the bed, the damage is more than just a bruise. Those injuries sometimes require the nail to be removed so the nail bed underneath can be examined and repaired.
Signs of Infection to Watch For
A black toenail from trauma can occasionally develop a secondary infection, especially if the nail is damaged or loose. Watch for pus draining from around the nail, increasing redness and swelling that spreads beyond the toe, or feeling feverish and shivery. These symptoms mean bacteria have gotten into the injured tissue and you need treatment. People with diabetes should be particularly attentive to any toenail injury, since foot infections can escalate quickly when circulation or nerve sensation is impaired.