A black nail is most often a bruise under the nail, caused by some kind of impact or pressure. In most cases it’s harmless and grows out on its own, but the discoloration can also signal a fungal infection, a nutritional deficiency, or, rarely, melanoma. The cause usually becomes clear based on how the dark spot appeared, whether it’s changing, and what other symptoms come with it.
Bruising Under the Nail
The most common cause of a black nail is a subungual hematoma, which is simply blood trapped between the nail and the nail bed. Stubbing your toe, dropping something on your finger, or wearing tight shoes during a long run can all burst tiny blood vessels underneath the nail. The pooled blood shows through as a dark red, purple, or black spot.
A mild bruise will grow out with the nail over time. Fingernails grow at roughly 3.5 millimeters per month, so a bruised fingernail can take about six months to look normal again. Toenails grow at less than half that speed (about 1.6 mm per month), meaning a black toenail from running or a stubbed toe may need a full year or longer to clear completely.
Pain from a nail bruise typically fades within a few days. If the pressure keeps building and the pain gets worse in the hours after the injury, a doctor can perform a quick drainage procedure called trephination to relieve it. You should also get medical attention if the nail is bleeding heavily, the finger or toe looks crooked, or you suspect a broken bone underneath.
Runner’s Toe and Repetitive Trauma
Distance runners, hikers, and athletes who make sudden stops (tennis, soccer, basketball) often develop black toenails without a single obvious injury. The nail repeatedly bumps the inside of the shoe during downhill stretches or quick direction changes, and over weeks of training, enough micro-damage accumulates to create a bruise.
Preventing it comes down to shoe fit. Your running shoes should leave about half an inch of space between your longest toe and the front of the shoe, enough room to freely wiggle your toes. Lacing techniques can help too. A “runner’s loop” locks the heel in place so the foot doesn’t slide forward, and window lacing reduces pressure points on the top of the foot. Getting fitted later in the day, when feet are slightly swollen, gives a more realistic sense of how the shoe will feel mid-run.
Fungal and Bacterial Infections
A nail that turns dark gradually, without any injury you can remember, may be infected. Fungal infections are the usual culprit. Certain molds and dermatophyte fungi can invade the nail plate and produce brown or black discoloration, often alongside thickening, crumbling edges, or a foul smell.
Bacteria can also darken a nail. A germ called Pseudomonas aeruginosa produces a pigment that turns nails green to dark green, sometimes dark enough to look nearly black. This tends to happen in nails that are already damaged or frequently exposed to moisture. Pseudomonas often co-infects alongside fungal organisms, creating a layered problem that may need both antifungal and antibacterial treatment.
Vitamin B12 Deficiency
Severely low vitamin B12 levels can trigger darkening of the nails, ranging from a bluish tint to distinct blue-black streaks running lengthwise. The mechanism involves melanin, the same pigment responsible for skin color. When B12 drops low enough, it reduces levels of a protective molecule called glutathione. Without enough glutathione, an enzyme involved in melanin production becomes overactive, and excess pigment gets deposited in the growing nail.
This type of nail change is uncommon and usually accompanied by other signs of B12 deficiency: fatigue, tingling in the hands or feet, difficulty with balance, or a sore tongue. Correcting the deficiency typically allows normal nail color to return as new nail grows in.
Medications and Chemotherapy
Certain medications can cause darkened nails as a side effect. Chemotherapy is the most well-known trigger. Patients receiving cancer treatment can develop increased pigmentation in the nail plate that persists even after treatment ends. The discoloration may appear as dark bands, patches, or an overall darkening across multiple nails, which helps distinguish it from a single-nail injury or infection.
Some non-cancer drugs, including certain antibiotics, antimalarials, and antiretrovirals, can produce similar changes. If dark nails appear shortly after starting a new medication, that timing is a strong clue.
Underlying Medical Conditions
Several systemic conditions can change nail color. Addison’s disease and Cushing syndrome both affect hormone levels in ways that increase melanin production, leading to dark longitudinal streaks. Hyperthyroidism and HIV infection can do the same. Kidney failure produces a distinctive pattern where the bottom half of the nail turns white and the upper half turns pink or darkened, sometimes called “half-and-half nails.”
In people with darker skin tones, vertical brown or black lines in the nails (called melanonychia striata) are often completely normal and present on multiple nails. This is a common variation that requires no treatment. Pregnancy and minor nail injuries can also produce these lines temporarily.
When a Black Nail Could Be Melanoma
Subungual melanoma, a type of skin cancer that starts under the nail, is rare but serious. It usually appears as a dark brown or black streak running from the base of the nail to the tip. Over time the streak may widen, become uneven in color, or cover more of the nail.
The most important warning sign is called the Hutchinson sign: pigment that spreads beyond the nail itself into the surrounding skin and cuticle. A bruise stays contained under the nail plate. Melanoma can leak color into the skin folds around the nail, and that distinction matters. Other red flags include a streak on only one nail (especially the thumb or big toe), a band that keeps getting wider, and discoloration that doesn’t grow out over several months the way a bruise would.
Subungual melanoma is more common in adults over 50 and disproportionately affects people with darker skin tones, partly because it can be mistaken for normal melanonychia and diagnosed late. Any new, single dark streak that progressively widens or shows pigment spreading into the cuticle warrants evaluation by a dermatologist. Early detection makes a significant difference in outcomes.
How to Tell the Difference
A few practical questions can help you narrow down what’s going on:
- Did you injure the nail recently? A bruise is the most likely explanation, and it should slowly migrate toward the tip as the nail grows.
- Is the nail thick, crumbly, or foul-smelling? Those point toward a fungal or bacterial infection.
- Are multiple nails affected? Medication side effects, nutritional deficiencies, and systemic conditions tend to affect more than one nail at a time.
- Is there a single dark streak that’s getting wider? That pattern, especially with pigment extending onto surrounding skin, needs a dermatologist’s assessment.
- Is the dark area growing out with the nail? Bruises and injuries move with the nail as it grows. A melanoma stays anchored near the base.