A black toenail almost always means blood has pooled underneath the nail after some kind of injury, a condition called subungual hematoma. Stubbing your toe, dropping something on it, or wearing tight shoes during a long run can all rupture tiny blood vessels in the nail bed, leaving a dark bluish-black patch that slowly grows out over weeks or months. That said, trauma is not the only explanation. Fungal infections, certain medications, bacterial colonization, and in uncommon but serious cases, melanoma beneath the nail can all turn a toenail dark. Figuring out which cause applies to you matters, because some require nothing more than patience while others need prompt medical attention.
Subungual Hematoma, the Most Common Culprit
When blood collects between the nail plate and the nail bed after an injury, the result is a subungual hematoma. The nail itself is translucent, so the trapped blood shows through as a dark blue, purple, or black discoloration that can cover part or all of the nail.1PubMed Central. Subungual Hematoma: Insights From a Clinical Case Study Crush injuries to the toe are the classic trigger, but repetitive microtrauma counts too. Distance runners are familiar with “runner’s toe,” where the toenail repeatedly jams against the front of the shoe during downhill stretches or long training sessions. The discoloration can appear within hours of an acute injury or build up gradually with repetitive impact.
Small hematomas often resolve on their own. The dark spot migrates forward as the nail grows, eventually reaching the free edge where it gets trimmed away. Toenails grow slowly, roughly a millimeter or so per month, so it can take six months to a year before the discoloration fully clears. Larger hematomas that cover more than about half the nail surface or cause significant throbbing pain may benefit from drainage. A clinician can puncture the nail with a heated needle or a large-gauge needle to release the trapped blood, which provides almost immediate pain relief.2The Journal of Emergency Medicine. Use of an 18-Gauge Needle for Subungual Hematoma
If a subungual hematoma is not drained when the pressure is high, the sustained force on the nail matrix can cause lasting damage. This sometimes shows up as ridging, splitting, or a permanent change in the nail’s shape.3PubMed Central. Subungual Hematoma So while the dark color itself is not dangerous, ignoring a painful, swollen hematoma can leave you with a nail that never quite grows back normally.
Fungal and Bacterial Infections
Not every black toenail traces back to a bump or a bruise. Fungal nail infections, or onychomycosis, are extremely common and typically turn nails yellow, white, or brown. Occasionally, though, a darker organism is involved. One documented case involved a black yeast-like fungus called Cladophialophora boppii infecting the toenail of an otherwise healthy woman with no immune deficiency. The same dark mold was isolated from her toenail on three separate occasions, confirming it as a genuine pathogen rather than a contaminant.4Medical Mycology. Toenail infection by Cladophialophora boppii Cases like this are unusual, but they illustrate that not all fungal nail infections look the same. If your toenail is thickened, crumbly at the edges, and progressively darkening without any history of trauma, a fungal cause is worth investigating.
Bacterial infection can also darken a nail. Pseudomonas aeruginosa, a common environmental bacterium, sometimes colonizes nails that are already damaged or chronically exposed to moisture. It typically produces a greenish discoloration, but the color range extends from greenish-yellow through greenish-brown to greenish-black. A dark nail with a greenish tint, especially if the surrounding skin is puffy or the nail is lifting from the bed, should raise suspicion for Pseudomonas. Treatment usually involves an oral antibiotic, and it tends to resolve well once addressed.5PubMed Central. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons
Fungal nail infections, even when they are not dangerous, can take a real toll on quality of life. Research has confirmed that toenail onychomycosis can reduce physical and social functioning and affect emotional well-being, with women and people who have had the infection for more than two years reporting the worst impact.6PubMed Central. Quality of life in patients with toenail onychomycosis That is worth knowing because many people delay treatment, assuming a discolored toenail is purely cosmetic. Getting it treated early can spare you months of self-consciousness and discomfort.
Medications That Darken Nails
Certain drugs can cause nail pigmentation as a side effect, and the resulting color can be dark enough to look alarming. Chemotherapy agents are probably the best-known offenders, but some antibiotics can do it too. One documented case involved a young woman who developed a diffuse blue-black discoloration on her thumbnail and big toenail about two weeks after finishing a course of sparfloxacin, a fluoroquinolone antibiotic. When she took the same drug a year later, the same nails darkened again in the same pattern, strongly suggesting a fixed drug reaction in the nail bed.7PubMed Central. Sparfloxacin-induced nail pigmentation: A case of fixed drug eruption?
Drug-induced nail discoloration typically affects multiple nails and tends to resolve once the medication is stopped, though it can take several months for the darkened portion to grow out. If you notice your toenails darkening after starting a new medication, bring it up with your prescriber. In most cases the pigmentation is harmless, but it can mimic more serious conditions, and your doctor may want to confirm the cause before letting it ride.
When to Worry About Melanoma
This is the cause that gets the most fear and, proportionally, the least understanding. Subungual melanoma is a type of skin cancer that develops in the nail matrix and usually shows up as a dark longitudinal streak running from the cuticle to the tip of the nail. It is uncommon in the overall population, but it accounts for a disproportionate share of melanoma in people with darker skin. In one case series, the subungual form represented roughly 2% of non-sun-induced melanomas in Western populations but up to 75% in African populations, about 25% in Chinese populations, and around 10% in Japanese populations.8PubMed Central. Subungual Melanoma is Not so Rare: Report of Four Cases from India
These numbers matter because the conventional wisdom that melanoma primarily affects fair-skinned people can delay diagnosis in darker-skinned individuals. Subungual melanoma of the big toenail is the most common presentation. In the same case series, all four patients had melanoma on the big toe, and two already had metastases to the liver and lymph nodes by the time they were diagnosed. That delay is the real danger: the cancer itself is treatable when caught early, but it is easy to dismiss a dark streak under a toenail as a bruise or a fungal infection for months or even years.
A few warning signs help distinguish subungual melanoma from a harmless hematoma or benign pigmentation. A dark band that widens over time, pigmentation that extends beyond the nail onto the surrounding skin (sometimes called the Hutchinson sign), and a streak that is present on only one nail without a clear history of injury all warrant evaluation by a dermatologist. A subungual hematoma tends to migrate forward with nail growth; melanoma-related pigmentation stays anchored in the same spot because the abnormal cells are in the nail matrix itself.
How Doctors Tell These Causes Apart
Distinguishing between a simple blood blister and something more worrisome is not always straightforward by eye alone. Dermoscopy, a technique that uses a magnifying instrument with polarized light to look at structures beneath the nail surface, can help substantially. Research has found that subungual hematomas show characteristic patterns under dermoscopy that differ from the patterns seen in nail-unit melanoma, making the tool valuable for narrowing the differential without an immediate biopsy.9British Journal of Dermatology. Dermoscopy of subungual haemorrhage: its usefulness in differential diagnosis from nail‐unit melanoma One study also identified previously undescribed features of subungual hematomas under dermoscopy, including a blue-white color that appeared in about a quarter of cases and was mostly located in the proximal third of the nail.10PubMed Central. Dermoscopic diagnosis of subungual hematoma: new observations
Newer imaging techniques like optical coherence tomography are also being studied. In one examination, a melanoma in situ under the nail showed irregularly sized and distributed curved blood vessels on the dermis of the nail bed, while benign hematomas and nevi showed clustered red dots or regularly distributed vessels instead.11PubMed. Examination of Subungual Hematomas and Subungual Melanocytic Lesions by Using Optical Coherence Tomography and Dermoscopy These advanced tools are not available in every clinic, but they illustrate that medicine is getting better at sorting out ambiguous nail pigmentation without jumping straight to surgical biopsy.
When imaging does not provide a definitive answer, a nail biopsy remains the gold standard. This involves removing a portion of the nail and sampling the tissue underneath. It is not a casual procedure, but if a dermatologist suspects melanoma, it is the only way to know for certain.
Benign Melanonychia and Why a Dark Streak Is Not Always Bad News
Melanonychia is the broad clinical term for brownish-black pigmentation of the nail plate, and its causes span a wide range. It can result from melanocyte activation triggered by local trauma, inflammatory conditions, systemic diseases, or medications. It can also come from melanocyte hyperplasia, which includes both benign growths and malignant tumors.12PubMed Central. Trauma-Associated Pseudo-Hutchinson Sign: An Autobiographical Case Report Emphasizing Conditions, Pseudo-Conditions, and Pseudo-Pseudo-Conditions A longitudinal dark band on a single nail understandably causes alarm, but the majority of melanonychia cases turn out to be benign.13PubMed Central. Melanonychia: Etiology, Diagnosis, and Treatment
People with darker skin tones are particularly likely to develop benign longitudinal melanonychia, sometimes on multiple nails simultaneously, as a normal variant of pigmentation. This kind of streak tends to be stable in width, uniform in color, and present on more than one nail. That profile contrasts sharply with subungual melanoma, which typically appears on a single nail, widens over time, and may show irregular pigmentation. Knowing this difference can save you unnecessary anxiety, though any new or changing dark streak still deserves a professional look.
Age-Related Nail Changes
Older adults are more prone to black toenails for several overlapping reasons. Nail disorders in general become more common with age. Elderly patients frequently develop brittle nails, thickened nails, splinter hemorrhages, and subungual hematomas as part of the normal aging process, compounded by changes in gait, footwear habits, and reduced peripheral circulation.14PubMed Central. Nails in systemic disease Faulty biomechanics and chronic low-grade trauma to the toes are especially common in this group.
At the same time, the risk of subungual melanoma increases with age. The case series mentioned earlier involved patients ranging from their forties to their seventies. Older adults who notice a new dark streak or an expanding area of pigmentation under a toenail should not write it off as just another part of aging. The threshold for getting a dermatologic evaluation should be lower, not higher, as you get older, precisely because the stakes of a missed melanoma increase.
Preventing Black Toenails in the First Place
For athletes and anyone who spends a lot of time on their feet, the most practical prevention strategy focuses on footwear. Shoes with a raised, roomy toe box reduce in-shoe displacement and deformation of the toes during activities like long-distance running, which in turn lowers the risk of repetitive toenail trauma.15PubMed Central. Toe Box Shape of Running Shoes Affects In-Shoe Foot Displacement and Deformation: A Randomized Crossover Study Keeping toenails trimmed short and straight across also helps, since a long nail catches more impact against the shoe.
Beyond shoes, broader training habits matter. Carefully planned training programs, proper surface selection, and protective footwear are all recognized injury prevention measures for runners.16PubMed Central. A guide to the prevention of running injuries Ramping up mileage gradually gives your feet time to adapt. If you routinely lose toenails during training cycles, that is a sign your shoes do not fit well, not a badge of toughness.
For fungal prevention, keeping feet dry, avoiding prolonged moisture exposure, and wearing breathable socks all reduce your risk. Pseudomonas infections are similarly linked to chronic moisture, so if your work or hobbies keep your feet wet for hours, drying them thoroughly and changing socks matters more than you might think.
When a Black Toenail Needs a Doctor
Most black toenails from trauma need no medical intervention at all. You can manage a mild subungual hematoma at home as long as the pain is tolerable, the toe is not deformed, and the discoloration is clearly linked to an injury you remember. Situations that do warrant a visit include:
- Severe pain or pressure: A large hematoma causing intense throbbing may need to be drained to prevent damage to the nail matrix.
- No trauma history: If you cannot recall injuring the toe and the dark color appeared spontaneously, the cause needs to be identified.
- A dark longitudinal streak: Particularly one that is widening, irregularly pigmented, or present on a single nail, this pattern should be evaluated to rule out melanoma.
- Pigment extending to the skin: Any darkening that spreads beyond the nail fold onto the surrounding skin warrants prompt dermatologic evaluation.
- Signs of infection: Redness, warmth, swelling, pus, or a greenish hue around the nail suggest bacterial involvement that may need antibiotics.
- Chronic thickening and crumbling: A dark, thick, crumbly nail without a clear trauma history may point to a fungal infection that will not resolve without treatment.
If you are uncertain, err on the side of getting it checked. A quick dermoscopy exam at a dermatologist’s office is painless and can often give you an answer on the spot. For most people, the reassurance alone is worth the visit. And for the small fraction whose black toenail turns out to be something more than a bruise, early detection changes outcomes dramatically.