A black toenail is almost always caused by blood pooling underneath the nail after an injury, a condition called a subungual hematoma. It’s the most common explanation by far, but fungal infections, reduced blood flow, certain medications, and rarely, melanoma can also darken a toenail. The cause matters because some of these are harmless and resolve on their own, while others need medical attention.
Injury and Repetitive Pressure
When something hits, crushes, or repeatedly presses on your toenail, the tiny blood vessels in the nail bed break open and bleed. Normally, the nail plate sits flush against the tissue beneath it with no gap. So the leaked blood has nowhere to go. It pools in that tight space, creating pressure that causes both the dark color and the throbbing pain you feel. Dropping a heavy object on your foot, stubbing your toe hard, or slamming it against furniture are classic triggers.
But you don’t need a single dramatic injury. Runners, hikers, and anyone who spends long hours on their feet can develop a black toenail from repetitive microtrauma. Shoes that are too tight or too short push against the nail with every step, and over miles or hours, that low-grade pressure is enough to cause bleeding underneath. This is sometimes called “runner’s toe,” and it most often affects the big toe or second toe.
A small, painless hematoma doesn’t need treatment. The discolored nail simply grows out over time. Toenails grow slowly, though. On average, it takes up to 18 months for a toenail to fully regrow from base to tip, so you may be looking at a dark nail for a while. If the hematoma covers more than half the nail and the pressure is painful, a doctor can relieve it by making a small hole in the nail to let the trapped blood drain. This is a quick in-office procedure that provides almost immediate relief.
Fungal and Bacterial Infections
Fungal nail infections typically start as a white or yellow discoloration near the edge of the nail, then gradually thicken and darken the nail as the infection progresses. In advanced cases, the nail can turn brown or black. These infections thrive in warm, moist environments like sweaty shoes, public showers, and pool decks. They tend to develop slowly over weeks or months rather than appearing overnight, which helps distinguish them from an injury.
Bacterial infections can also discolor nails. One type of bacteria produces pigments that turn the nail green or black, usually after the nail has already been damaged or lifted slightly from the nail bed, giving bacteria a way in. If you notice discoloration along with swelling, warmth, discharge, or a foul smell, an infection is the likely culprit. Fungal infections are treated with antifungal medications, which can take months to work because the nail grows so slowly. Bacterial infections typically need a different approach and should be evaluated sooner.
Reduced Blood Flow and Diabetes
Black toenails can signal a more serious circulation problem, especially in people with diabetes or peripheral artery disease (PAD). PAD narrows the blood vessels that carry blood from the heart to the legs, which means your toes don’t receive enough oxygen and nutrients. Over time, this reduced blood flow can cause tissue damage that shows up as darkened or blackened toenails and skin.
In severe cases, PAD leads to gangrene, where tissue actually dies from lack of blood supply. Gangrene usually starts at the extremities, including the toes, and can involve skin discoloration, swelling, and discharge. This is a medical emergency.
Diabetes adds another layer of risk through nerve damage in the feet, called diabetic neuropathy. When you can’t feel your toes well, you’re far more likely to injure them without realizing it. A toe that’s been bumped, compressed, or cut may go unnoticed for days, and the resulting hematoma or infection develops unchecked. This is a common reason people with diabetes discover blackened toenails seemingly out of nowhere.
Medications That Darken Nails
Certain drugs can activate the pigment-producing cells in the nail bed, causing brown or black streaks (longitudinal bands running from base to tip) or darkening of the entire nail. Chemotherapy agents are the most well-known culprits, particularly drugs like cyclophosphamide, hydroxyurea, and doxorubicin. But other medications can do this too: some HIV drugs, psoriasis treatments, antimalarials, and certain antibiotics have all been linked to nail color changes.
If your toenails darkened after starting a new medication, that’s likely the connection. The discoloration is usually cosmetic and reversible once the medication is stopped or changed, though it takes months for the affected nail to grow out completely.
When to Take It Seriously
Most black toenails are harmless hematomas that resolve on their own. But a few patterns should prompt a closer look. A dark streak or spot that appears without any injury, grows wider over time, or extends beyond the nail onto the surrounding skin could be subungual melanoma, a rare but serious form of skin cancer that develops under the nail. This type of discoloration doesn’t grow out with the nail the way a hematoma does. Instead, it stays in place or expands.
You should also pay attention if the nail begins separating from the nail bed, if you see signs of infection like pus or increasing redness, or if the blackening appeared suddenly with no obvious cause. People with diabetes or known circulation problems should treat any toenail color change as worth investigating, since what looks minor on the surface can reflect deeper tissue damage.
Preventing Black Toenails
If repetitive pressure is the cause, the fix is almost always better-fitting shoes. Podiatrists recommend leaving about half an inch of space between the tip of your longest toe and the end of the shoe. The toe box should be wide enough that you can wiggle your toes freely. Feet swell during exercise, so shoes that feel fine when you try them on in a store may be too tight after a few miles of running. If your foot slides forward inside the shoe, try relacing to secure the midfoot more snugly, keeping pressure off the toes. Moisture-wicking socks and keeping toenails trimmed short (but not too short) also reduce the friction that leads to bruising over time.