A black toenail almost always means blood has pooled underneath the nail after some kind of injury. This collection of trapped blood, called a subungual hematoma, accounts for the vast majority of cases and is rarely dangerous. But the same dark discoloration can occasionally signal something more serious, from a fungal infection producing dark pigment to, in rare cases, a type of skin cancer called subungual melanoma. Figuring out which cause you’re dealing with usually comes down to a few practical clues.
Subungual Hematoma Is the Most Common Cause
When you stub your toe, drop something heavy on your foot, or jam your toes against the front of a shoe, tiny blood vessels beneath the nail can rupture. Blood leaks into the space between the nail plate and the nail bed, and because it has nowhere to go, it pools there. At first it may look red or dark purple; over the following days it typically darkens to blue-black or solid black. The pressure from this trapped blood is what causes the throbbing pain many people feel after the injury.
You don’t always need a single dramatic event to end up with a black toenail. Repetitive, low-grade pressure produces the same result through accumulated microtrauma. Runners, hikers, and athletes in sports that involve sudden stops are especially prone to this. The condition is common enough in distance runners that sports medicine literature refers to it simply as “black toe,” caused by repeated bleeding under the nail from the toe bumping against the shoe with each stride.1PubMed Central. Common cutaneous disorders in athletes Shoes that are too tight, too loose, or too short in the toe box are the usual culprits. Downhill running makes it worse because your foot slides forward with each step.
A subungual hematoma from a single acute injury is usually easy to recognize: you remember the moment it happened, the pain was immediate, and the dark area doesn’t extend beyond the nail itself. Hematomas from repetitive microtrauma can be trickier because they develop gradually and may not hurt much. In either case, the discoloration stays within the nail and grows out with it over weeks to months. If you watch the dark patch, you’ll see it slowly migrate toward the tip of your toe as the nail grows, eventually reaching the free edge where it can be trimmed off.
Relieving the Pressure
When a subungual hematoma is fresh and painful, the standard treatment is trephination, a procedure where a doctor creates a small hole through the nail plate to let the pooled blood drain out. This relieves the pressure almost immediately. A study evaluating a controlled drilling technique found that patients experienced substantial pain relief over the eight hours following the procedure, with minimal discomfort during the process itself and little risk of damaging the nail bed underneath.2PubMed. Controlled nail trephination for subungual hematoma
If the hematoma is small and not especially painful, treatment is often optional. The blood reabsorbs on its own over time, and the discolored nail grows out naturally. This can take anywhere from six to nine months for a toenail, since toenails grow much more slowly than fingernails. During that time, the nail may look cosmetically unappealing but is otherwise harmless. One thing worth noting: if the trauma was severe enough to cause a hematoma covering more than half the nail, there’s a higher chance of an underlying injury to the nail bed or even a fracture of the distal phalanx (the small bone at the tip of the toe). In those cases, a doctor may want to remove the nail to inspect and repair the bed beneath it.
When a Dark Toenail Could Be Melanoma
Subungual melanoma is the cause that worries people most, and for good reason: it’s a form of skin cancer that can be aggressive if caught late. The reassuring news is that it’s uncommon. But it does happen, and it can be easy to dismiss as “just a bruise” because the earliest visible sign is often a dark streak or band running the length of the nail.
This dark longitudinal streak, called longitudinal melanonychia, was present in all 18 cases of early subungual melanoma examined in one pathology study. The streaks consisted of irregular dark brown to black lines within a brown background.3PubMed. Scattered atypical melanocytes with hyperchromatic nuclei in the nail matrix: diagnostic clue for early subungual melanoma in situ That irregularity is key. A benign mole (nevus) under the nail can also produce a dark longitudinal band, but the lines in a benign streak tend to be uniform in color, spacing, and width. When melanoma is the source, the lines are uneven: they vary in thickness, color intensity, and spacing.
Another important warning sign is called Hutchinson’s sign, where the dark pigment extends beyond the nail plate itself onto the surrounding skin of the nail fold or fingertip. A large comparative study found Hutchinson’s sign in about 83% of subungual melanoma cases. In those melanoma cases, the pigmentation on the surrounding skin tended to extend beyond half the nail width, was often wider than the streak on the nail itself, and appeared discontinuous or patchy. By contrast, a benign look-alike called pseudo-Hutchinson’s sign can occur with harmless longitudinal melanonychia. The benign version has a distinctive pattern: a neat linear lateral border, pigment that fades as it moves toward the base of the nail, and pigmentation that disappears under magnification with a dermatoscope.4PubMed. Clinical differences between Hutchinson’s sign in subungual melanoma and pseudo-Hutchinson’s sign in benign longitudinal melanonychia
The practical takeaway: a subungual hematoma typically appears as a solid, roughly round area of discoloration that tracks outward as the nail grows. Melanoma tends to appear as a lengthwise stripe that stays in the same location because the abnormal cells sit in the nail matrix where new nail is being produced. If you have a dark streak under your toenail that hasn’t followed an injury, hasn’t grown out after several months, or is getting wider, you should have a dermatologist look at it. People with darker skin tones are at higher risk for subungual melanoma and are also more likely to have benign longitudinal melanonychia, which makes professional evaluation even more important for distinguishing the two.
Fungal Infections That Turn Nails Dark
Most people associate fungal nail infections with nails that turn yellow, thick, and crumbly. That’s the typical presentation. But a subset of fungal infections produce dark brown-to-black pigmentation of the nail, a condition known as fungal melanonychia. The organisms responsible are often dematiaceous fungi, a group of molds that naturally produce dark pigment (melanin) as part of their biology. A review in the dermatology literature identified over 21 species of these pigment-producing fungi and at least 8 species of non-pigmented fungi that can also cause dark nail discoloration.5PubMed. Fungal melanonychia
Fungal melanonychia is considered relatively rare compared to standard onychomycosis, but it may be underdiagnosed because the dark color can lead clinicians to suspect melanoma or a hematoma first. The discoloration tends to develop gradually and may involve thickening or textural changes to the nail over time. If your toenail has turned dark and also looks thickened, rough, or is separating from the nail bed, a fungal infection is a strong possibility. A doctor can clip a piece of the nail and send it for culture or microscopy to identify the organism.
Bacterial Infections and Green-Black Nails
A less well-known cause of dark nail discoloration involves bacteria rather than fungi. Pseudomonas aeruginosa, a common environmental bacterium, can colonize the space under a nail that has already been damaged or lifted (a condition called onycholysis). Pseudomonas produces a bluish-green pigment called pyocyanin, which stains the nail. The resulting color ranges from pale green to greenish-black, and in some cases the nail can look nearly black.6PubMed. Green nail: Etiology and treatment of chloronychia
This condition, formally called chloronychia or green nail syndrome, typically shows up in people whose hands or feet are frequently wet, exposed to soaps and detergents, or subjected to chronic mechanical irritation. The classic triad includes greenish discoloration of the nail plate, chronic swelling of the tissue around the nail (paronychia), and separation of the nail from the bed at the tip or sides. In elderly patients, green or black nail discoloration should raise suspicion for Pseudomonas and can be treated with an oral antibiotic.7PubMed Central. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons Keeping the nail dry and avoiding prolonged moisture exposure is equally important for clearing the infection.
Medications That Darken Your Nails
A number of prescription drugs can cause nail discoloration as a side effect, and the color change can range from brownish bands to outright black. Among the most common culprits are cancer chemotherapy drugs, retinoids, and certain antiretroviral medications used to treat HIV. Drug-induced nail changes tend to affect several nails at once rather than just one, which is a useful clue that a medication rather than a localized injury or infection is responsible.8PubMed. Drug-induced nail abnormalities
Drug-induced pigmentation of the skin, nails, and mucous membranes is estimated to account for about a fifth of all cases of acquired hyperpigmentation, and over 50 different agents have been implicated. The list includes antibiotics, antimalarials, antipsychotics, and prostaglandin analogs (sometimes used in glaucoma eye drops), among others.9PubMed. Drug-Induced Pigmentation: A Review The timing is often a giveaway: if your nails started darkening weeks to months after beginning a new medication, the drug is a likely suspect. In most cases, the pigmentation is reversible once the offending medication is stopped, though it can take many months for the discolored portion of the nail to grow out completely.
How Doctors Tell These Causes Apart
When a dermatologist evaluates a dark toenail, the first tool after a clinical history is a dermatoscope, essentially a specialized magnifying device with polarized lighting. Dermoscopy can often distinguish between a hematoma, a benign nevus, and melanoma without any cutting.
In a study of over 100 pigmented nail lesions, dermoscopy revealed specific patterns for each cause. Subungual hemorrhages (hematomas) showed a well-defined, rounded proximal edge with purple-to-brown coloring in every single case examined. Benign melanocytic nevi showed a brown background with regular longitudinal lines that were consistent in their thickness, spacing, and color. Melanoma, by contrast, showed brown-to-black lines that were irregular in color, spacing, thickness, and alignment. The irregular pattern was significantly associated with melanoma compared to all other diagnoses.10Archives of Dermatology. Dermoscopic Examination of Nail Pigmentation
If dermoscopy raises enough suspicion for melanoma, the next step is a nail biopsy. This involves sampling tissue from the nail matrix, which is the growth center at the base of the nail where pigment-producing cells reside. Several biopsy techniques exist, including punch biopsy, excision biopsy, and shave biopsy, and the choice depends on the size and location of the lesion. Patients should know that the procedure carries some risks, including temporary discomfort, the time it takes for the nail to regrow, and a small chance of permanent nail irregularity.11PubMed Central. Nail Biopsy: A User’s Manual Even with those caveats, biopsy is the only way to definitively rule melanoma in or out when the clinical picture is ambiguous.
A Quick Guide to Sorting Out the Clues
Because so many different conditions can turn a toenail black, context matters more than the color itself. A few questions can help you narrow things down before you ever see a doctor:
- Was there an injury? If you can trace the discoloration to a specific trauma or to a period of heavy running or hiking, a hematoma is the most likely explanation. Watch it over a few weeks: the dark spot should migrate toward the tip of the nail as it grows.
- Is the nail thickened or crumbly? Textural changes alongside dark pigment point toward a fungal infection, especially if the nail is also separating from the bed.
- Is the color a streak or a spot? A lengthwise stripe that stays in place while the rest of the nail grows is more concerning for a melanocytic process (benign or malignant) than a round blotch that moves with nail growth.
- Are multiple nails affected? Darkening across several nails at once suggests a systemic cause like a medication side effect rather than a localized injury or tumor.
- Is there pigment on the skin around the nail? Pigment spreading onto the surrounding cuticle or skin (Hutchinson’s sign) is a red flag for melanoma and warrants prompt evaluation.
None of these clues is perfectly reliable on its own. A hematoma can occasionally mimic a melanoma streak if it occurs near the nail fold, and an early melanoma can look deceptively like a benign dark band. When in doubt, a dermatologist equipped with a dermatoscope can usually sort things out in a single visit.
Benign Longitudinal Melanonychia
Not every dark lengthwise streak on a toenail is melanoma. Longitudinal melanonychia, where a brown-to-black band runs from the base to the tip of the nail, is common and in most cases entirely benign. It can be caused by a mole (nevus) in the nail matrix, activation of the nail’s melanocytes from friction or repeated trauma, or ethnic variation in melanin production. This type of pigmentation is especially prevalent in people with darker skin; studies estimate that the majority of Black adults have some degree of longitudinal melanonychia on at least one nail.12PubMed Central. Melanonychia: Etiology, Diagnosis, and Treatment
The challenge is that benign longitudinal melanonychia and early subungual melanoma can look similar to the naked eye. The features that help distinguish them, as described earlier, involve the regularity of the lines, the stability of the band’s width over time, and the presence or absence of Hutchinson’s sign. For people who have had a stable, unchanged dark band on a nail for years, the odds overwhelmingly favor a benign cause. But any band that is new, changing in width, becoming darker, or developing irregular borders deserves clinical attention. Dermatologists sometimes recommend photographing the nail every few months so that any subtle changes become easier to detect over time.
Age-Related Considerations
Both ends of the age spectrum bring their own quirks when it comes to dark toenails. In children, longitudinal melanonychia is relatively uncommon in lighter-skinned populations but does occur. When it appears in a child, the differential still includes a benign nevus, and pediatric subungual melanoma is exceedingly rare. Dermatologists tend to monitor these cases closely rather than immediately biopsy, since the ratio of benign to malignant cases is heavily skewed toward benign in younger patients.
In older adults, the picture shifts. Toenails become thicker, more ridged, and more prone to damage with age. Repeated low-grade trauma from shoes becomes more significant as nail structure weakens and foot shape changes over the years. Circulatory problems common in older people can also slow healing and make hematomas take longer to resolve. At the same time, the cumulative risk of melanoma increases with age, so a new dark streak appearing on an elderly person’s toenail arguably calls for faster evaluation than the same finding in a teenager. Fungal infections are also far more common with age, and the overlap of a thick, darkened, crumbly nail in an older person can make it harder to tell fungal disease from something more concerning without laboratory testing.
Shoes, Sports, and Prevention
For the large number of people whose black toenails come from trauma, prevention is mostly about footwear. Running shoes should leave about a thumb’s width of space between the longest toe and the end of the shoe. Lacing techniques that keep the foot from sliding forward during downhill sections can make a real difference for trail runners and hikers. Moisture-wicking socks reduce friction. Keeping toenails trimmed short and straight across prevents the nail edge from catching inside the shoe.
Athletes who repeatedly lose toenails or develop chronic hematomas sometimes develop permanent changes to the nail matrix over time, resulting in a nail that grows back thicker, ridged, or slightly discolored even after the blood has cleared. This isn’t dangerous, but it can be a cosmetic nuisance. In extreme cases, chronic damage to the nail bed can lead to the nail not regrowing fully at all, leaving a smaller or irregularly shaped nail. The simplest fix is also the most boring: wearing shoes that actually fit, and replacing worn-out athletic footwear before the toe box compresses.