A black toenail is almost always caused by blood trapped underneath the nail, a condition called a subungual hematoma. This is by far the most common explanation, especially if you recently stubbed your toe, dropped something on it, or have been running or hiking. Less often, a black toenail can result from a fungal infection, certain medications, or, rarely, a type of skin cancer that develops under the nail.
Trauma and Repetitive Pressure
When something injures the blood vessels in your nail bed, they leak blood into the tight space between the nail bed and the hard nail plate above it. That blood has nowhere to go. It pools, creating pressure that causes both the dark discoloration and the throbbing pain you might feel. A single forceful event, like dropping a heavy object on your foot, can cause this instantly. But so can repetitive, low-grade pressure over days or weeks.
Runners and hikers are especially prone to this. If your shoes are slightly too small or your foot slides forward with each step, the front of the shoe pushes against your toenails thousands of times per outing. The big toe and second toe take the worst of it. The discoloration might not appear until hours or even a day after the activity, which can make it harder to connect the dots.
What Happens as It Heals
A small, painless bruise under the nail typically resolves on its own. The dark spot gradually grows out with the nail over several months. If the hematoma is large, covering more than half the nail bed, and painful, a doctor can relieve the pressure by making a small hole in the nail to drain the trapped blood. This works best within the first day or two, before the blood clots. After that window, drainage is no longer effective and you’re better off letting it grow out naturally.
In more severe cases, the nail may loosen and eventually fall off. This looks alarming but is usually harmless. A new nail will grow in from the base, though toenails grow slowly. Full regrowth takes 6 months to 2 years after an injury, with 18 months being a reasonable average to expect.
Fungal and Bacterial Infections
If you haven’t injured your toe and the discoloration developed gradually, a fungal or bacterial nail infection is a likely culprit. The most common fungal infections are caused by organisms called dermatophytes, though yeast and molds can also be responsible. Bacterial infections tend to produce green or black discoloration specifically.
These infections thrive in warm, moist, dark environments, which describes the inside of most shoes perfectly. Gym showers, pool decks, and shared locker room floors are common places to pick them up. You’ll often notice other changes alongside the color shift: the nail may become thicker, crumbly, or slightly lifted from the nail bed. Fungal nail infections don’t resolve on their own and typically require treatment that can take several months to work, since the medication needs time to grow out with the new nail.
Medications and Systemic Causes
Certain drugs can trigger dark pigmentation in the nails. Chemotherapy medications, some antivirals, and a treatment used for conditions like sickle cell disease can all stimulate the pigment-producing cells in the nail matrix, resulting in brown or black bands or streaks that run lengthwise along the nail. If you’ve recently started a new medication and notice nail color changes, that connection is worth raising with your prescribing doctor.
When a Dark Streak Could Be Melanoma
This is the cause people worry about most, and while it’s genuinely rare, it’s worth understanding. Subungual melanoma, a type of skin cancer that develops under the nail, typically looks different from a bruise. Instead of a broad, uniform dark patch, melanoma usually appears as a single dark band running from the base of the nail to the tip. Over weeks to months, that band widens and its edges become irregular rather than staying crisp and parallel.
One important distinguishing feature: if the dark pigment extends beyond the nail itself onto the surrounding skin of the cuticle or fingertip, that’s a sign called Hutchinson’s sign, and it strongly suggests melanoma rather than a bruise. A bruise also tends to shift position as the nail grows out, while a melanoma streak stays anchored in place because the pigment is being continuously produced at the nail’s root.
Doctors evaluate suspicious nail pigmentation using what’s known as the ABCDEF framework: your age, the band’s width and border characteristics, changes in size or shape over time, which digits are affected, whether pigment has extended to surrounding skin, and your family history of melanoma. If any of these factors raise concern, a biopsy of the nail and underlying tissue confirms or rules out the diagnosis.
A black toenail that appeared without any injury, doesn’t grow out over several months, or keeps getting wider deserves a professional evaluation. The same applies if you notice pigment spreading to the skin around the nail.
Preventing Black Toenails
If repetitive pressure is the problem, the fix is almost always about shoe fit. Get your feet measured at a specialty running or shoe store to make sure your toe box gives your toes enough room. Your longest toe should have roughly a thumb’s width of space between it and the front of the shoe. Feet swell during exercise, so shoes that feel fine when you first put them on can become too tight miles into a run.
Lacing technique matters too. If your foot slides forward inside the shoe, adjusting the lacing pattern to lock the heel in place reduces how much your toes bang against the front. Moisture-wicking socks help by keeping feet drier, which reduces the slipping and friction that contribute to nail trauma. Keeping toenails trimmed short and straight across also removes some of the surface area that catches pressure from the shoe.