A black toenail is almost always caused by blood trapped under the nail after an injury or repeated pressure. This is called a subungual hematoma, and it’s the most common explanation by far. Less often, black discoloration comes from a fungal infection, certain medications, or external staining. Rarely, it signals something more serious like melanoma under the nail.
Bruising Under the Nail
When you stub your toe, drop something heavy on it, or wear tight shoes for hours, blood vessels in the nail bed get damaged and start leaking. Normally, the nail plate sits flush against the tissue beneath it with no gap. So when blood leaks, it has nowhere to go. It pools in that tight space, creating pressure that causes both the dark color and the throbbing pain you feel.
The discoloration typically starts as red or dark purple and shifts to black or dark brown over the following days. A key feature of a bruise under the nail: it stays in one spot and gradually moves toward the tip of your toe as the nail grows out. Toenails grow at roughly 1.6 mm per month, so a bruise near the base of a big toenail can take 12 to 18 months to fully disappear. During that time, the dark patch slowly inches forward until it reaches the free edge and you trim it off.
Not every bruised nail needs treatment. If the pain is mild and manageable, you can simply wait it out. But if the blood keeps expanding, the pain gets worse instead of better, or the nail starts lifting or splitting, a doctor can drain the blood by making a small hole in the nail plate. This works best within 24 to 48 hours of the injury. Left untreated, a large hematoma can cause permanent changes to your nail’s shape or texture, or you may lose the nail entirely.
Runner’s Toe and Repetitive Pressure
You don’t need a single dramatic injury to get a black toenail. Runners, hikers, and anyone who spends long hours on their feet in poorly fitting shoes can develop one from repetitive microtrauma. Each stride pushes the toe into the front of the shoe, and over miles or hours, that constant contact damages the nail bed just like a single blow would.
The fix is almost always about shoe fit. Your running shoes should have about a thumb’s width of space between your longest toe and the front of the shoe. Lacing matters too. If your laces are too loose near the ankle, your foot slides forward with every step, jamming the toes into the toe box. A heel lock lacing technique, where you thread the laces through the extra holes at the top of running shoes to create a loop on each side before tying, keeps the heel anchored and stops that forward slide. If you have high arches, skipping the lace holes near the top of the foot where the prominence sits can reduce pressure without sacrificing overall snugness.
Fungal Infections
Nail fungus is usually associated with yellow or white discoloration, but certain organisms, particularly bacteria and molds rather than the more common dermatophyte fungi, can turn a nail green or black. The color alone isn’t enough to identify a fungal infection, though. Look for a cluster of changes: the nail thickens, becomes brittle or crumbly, develops a rough or misshapen appearance, starts separating from the nail bed, or smells noticeably bad.
Fungal nail infections don’t resolve on their own. Over-the-counter antifungal treatments work for mild cases, but infections that have spread deep into the nail or across multiple toes typically need prescription-strength medication. Treatment can take months because the nail has to grow out completely before it looks normal again.
Medications That Darken Nails
Several types of medications can cause dark discoloration under or within the nail plate. Blood thinners and antiplatelet drugs can trigger small bleeds beneath the nail that look like splinter-shaped dark lines. Certain cancer treatments, particularly taxane-based chemotherapy, are well known for causing nail darkening. Some antibiotics in the tetracycline family (minocycline is a common one) can also deposit pigment in the nails. If your toenails darkened after starting a new medication, that connection is worth raising with your prescriber.
External Staining
Sometimes the black isn’t coming from inside the nail at all. Hair dye, certain topical antiseptics, and dark nail polish (especially worn without a base coat) can stain the nail plate itself. Silver nitrate, used in some medical and cosmetic products, reacts with proteins in the nail to form a black-brown metallic deposit that can be very stubborn to remove. The giveaway with external staining is that the color sits on the surface of the nail rather than beneath it, and it often affects the surrounding skin too.
When Black Toenails Signal Melanoma
Subungual melanoma is rare, but it’s the reason a persistent dark streak on a toenail deserves attention. This type of skin cancer develops from pigment-producing cells in the nail bed and most commonly affects the big toe. It typically appears as a dark brown or black vertical stripe running from the base of the nail toward the tip.
Several features help distinguish melanoma from a simple bruise. A bruise tends to appear as a blotchy patch that fades at the edges and slowly grows out with the nail. Melanoma produces a streak pattern, and that streak stays anchored at the base of the nail because the cancer cells keep producing pigment. In one study comparing the two, 100% of melanocytic lesions showed a streak pattern, while only 37% of bruises did. Bruises also tended to show multiple colors (reds, purples, browns), while melanocytic lesions were consistently brown.
Other warning signs include a stripe that widens over time, uneven pigmentation within the band, the nail cracking or becoming deformed, or the skin around the nail (the cuticle and nail folds) developing dark discoloration. That last sign, where pigment bleeds into the surrounding skin, is called the Hutchinson sign and is one of the strongest clinical red flags for nail melanoma.
A definitive diagnosis requires a biopsy, where a small tissue sample is taken from beneath the nail and examined under a microscope. Dermatologists also use a handheld magnifying tool called a dermatoscope to examine the nail’s pigment pattern in detail before deciding whether a biopsy is needed.
How to Tell What’s Causing Yours
Start with the simplest question: did something happen to this toe recently? If you can trace the discoloration back to an injury, a long run, new shoes, or a stubbed toe, a bruise is overwhelmingly the most likely explanation. Watch it over the next few weeks. A bruise will stay put or migrate toward the tip as the nail grows. It won’t expand backward toward the cuticle.
If the black appeared gradually with no clear trigger, or if it looks like a defined stripe rather than a blotch, pay closer attention. A stripe that has been present for weeks without any change in position, one that seems to be getting wider, or one accompanied by nail deformity or skin discoloration around the nail warrants a visit to a dermatologist. The same applies if a presumed bruise hasn’t moved or faded at all after two to three months, since even slow-growing toenails should show some visible shift in that time frame.
Thick, crumbly, foul-smelling nails point toward fungal infection regardless of color. And if you recently started a new medication or have been using dark nail products, consider those as potential explanations before assuming something more serious.