Why Did My Nail Turn Black? Causes and When to Worry

A nail that turns black is usually bruised, with blood pooled beneath the hard nail after a knock, a crush, or repeated rubbing. In most cases the dark patch is harmless and leaves as the nail grows out. But a dark mark is not always blood. Pigment made in the nail’s root, germs, medicines, and things that stain the nail can look much the same.1PubMed Central. Use of Nail Dermoscopy in the Management of Melanonychia: Review The useful question is less what color it is than what shape it has, how it started, and whether it moves as the nail grows.

The Most Common Cause Is Blood Under the Nail

Slamming a finger in a door or drawer, or taking a heavy blow to the nail, commonly bleeds into the space beneath it.2Harvard Health. Nail Trauma Repeated pressure causes the same thing. In running, the toes hit the front of the shoe over and over.3Quality in Sport. Mechanical and Friction-Induced Dermatoses in Athletes: Black Heel, Piezogenic Papules, and Related Traumatic Skin Conditions – a narrative review Because the nail is rigid, the trapped blood can press hard on the tissue underneath and throb badly during the first day.4PubMed. Controlled nail trephination for subungual hematoma

Not every bruise comes from a moment you remember. Tight shoes, a long hike, or repeated small knocks can bruise a nail without one memorable event.

The first days follow a fairly predictable course. Pain and throbbing are worst in the first hours to a day, then ease over the following days. Fresh blood usually looks red or purple, and as it dries and breaks down it turns darker, often brown or black. A nail that looks darker on day three than on day one is behaving like a normal bruise. The size of the patch usually reflects how much bleeding happened at the time of injury, and it tends not to keep enlarging afterward.

Blood tends to look like a red, purple, or brown-black blob of fairly even color that fades at its edges. It lacks the neat lengthwise lines that pigment streaks have.5PubMed Central. Use of Nail Dermoscopy in the Management of Melanonychia: Review

How Long a Black Toenail Takes to Clear

Blood under the nail usually clears by itself, moving toward the tip as the nail grows.6PubMed Central. Clinical and Dermoscopic Features of Subungual Hemorrhage: A Retrospective Study of 420 Cases Fingernails grow about 3.5 mm a month, while toenails grow about 1.6 mm a month.7PubMed. Growth rate of human fingernails and toenails in healthy American young adults A mark near the cuticle on a big toe can therefore take most of a year to reach the tip.

You can estimate your own timeline: measure from the dark edge to the free tip of the nail, then divide by the monthly growth figure. Because the big toenail grows slowly, the answer is often many months, so slow clearing is not a problem by itself.

Movement is the best home check. Because blood rides out with the nail, a bruise should slowly shift away from the cuticle, with a clear gap of normal nail behind it. Snapping a photo next to a ruler, or noting the distance from the cuticle each month, shows whether the mark is moving. A mark that stays rooted at the cuticle, or a streak that keeps running from the base, isn’t behaving like a bruise.

A few things are common while it grows out. The nail may lift and fall off, and a new one usually grows in behind it. Keep a loose nail trimmed and covered so it doesn’t catch on socks or towels. Nail polish can hide changes, so leave it off while you’re monitoring. If the nail was badly injured, the new one may come in slightly ridged or uneven, which is a scar from the injury and not a sign of disease. Some faint discoloration can also linger at the tip near the end.

Dark Streaks That Are Usually Harmless

A brown or black stripe running the length of the nail comes from pigment cells in the nail root becoming active or multiplying.8Journal of Onychology and Nail Surgery. Benign longitudinal melanonychia in skin of color: A case series Unlike blood, it grows out continuously from the cuticle, because the root keeps making it. A stripe has parallel edges running from cuticle to tip, while a smudge is roundish and may sit mid-nail. Look in good light and compare with your other nails.

These streaks are common in people with darker skin, found in about 1% of white people, 10 to 20% of Japanese and other Asian people, and 77 to 100% of African Americans.9PubMed Central. Melanonychia: Etiology, Diagnosis, and Treatment They usually show on several nails, especially thumbs and index fingers, and widen gradually with age.10PubMed Central. Melanonychia: Etiology, Diagnosis, and Treatment

Other harmless reasons exist too. A benign mole in the nail root can make one steady band. Pregnancy and some medicines can activate pigment cells, and frequent biting or picking can darken the nail root. Even so, a single band needs a doctor’s judgment, because moles and early cancer look alike. A common outcome is reassurance with photographs and follow-up, so seeing a doctor does not mean surgery is likely.

When a Black Nail Could Be Melanoma

Nail melanoma most often first shows up as a dark lengthwise streak, not a bruise-like blob.11Cancer. Clinical and histopathologic characteristics of early lesions of subungual malignant melanoma It is most common in the big toe and in people around their late 50s.12The American Journal of Surgical Pathology. Subungual Melanoma It makes up as much as a third of melanomas in African American, Asian and Native American people.13PubMed. The ABC rule for clinical detection of subungual melanoma

These features raise the concern:

Melanoma gets mistaken for a bruise because people assume a forgotten injury and wait. Unlike a bruise, it doesn’t grow out and leave clear nail behind. Many nail melanomas are found late; in one group most were already deep at diagnosis.23The American Journal of Surgical Pathology. Subungual Melanoma

The warning signs are clues, not a test. No single feature proves melanoma, and most dark streaks with one or two of them turn out to be harmless. Pigment on the surrounding skin isn’t proof either, since harmless causes can mimic it.24Actas Dermo-Sifiliográficas. Is Hutchinson’s Sign Pathognomic of Subungual Melanoma?25PubMed. Clinical differences between Hutchinson’s sign in subungual melanoma and pseudo-Hutchinson’s sign in benign longitudinal melanonychia The more features that line up, such as a single wide band that is changing in an adult, the stronger the reason to be seen promptly. Only a doctor’s examination, and sometimes a biopsy, can say what a streak really is.

At home, check all ten fingernails and toenails in good light. Photograph the streak next to a ruler every month or two, because slow widening is hard to notice from memory, and look at the skin beside the nail for spreading color.

Fungus, Bacteria, Medicines and Stains

Some molds make a dark pigment and can infect a nail, turning it brown to black.26Plasticsurgerykey.com. Fungal melanonychia A germ that thrives on wet hands can make nails green to black, often with the nail lifting and the fold swollen.27PubMed Central. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons People whose hands are often in water or soap, especially older adults, are most prone, and it is treated with antibiotic pills.28PubMed Central. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons It tends to arise when the skin fold has been damaged or the nail has lifted, letting germs settle in the moist gap. Keeping hands dry and wearing gloves with a cotton liner for wet work helps prevent a repeat, though a doctor decides the treatment.

Fungal darkening differs from a bruise because it changes the nail’s texture as well as its color, making it thicker, crumbly, or lifting from the nail bed. A bruise usually leaves the surface smooth and hard. Fungus does not clear on its own the way blood does, and it tends to spread slowly across the nail over time.

Cancer chemotherapy often darkens nails, as bands across or along the nail or all over, usually 1 to 2 months after starting.29PubMed Central. Nail Changes With Chemotherapeutic Agents and Targeted Therapies Drugs most linked to it include cyclophosphamide, doxorubicin and hydroxyurea.30Case Reports in Dermatology. Chromonychia Secondary to Chemotherapy These changes usually fade once treatment ends.31PubMed. Nail toxicities induced by systemic anticancer treatments Medicine-related darkening is usually painless and shows up on several nails at once, which separates it from an injury to a single nail. If you started a new drug a month or two before your nails darkened, tell the prescriber, but don’t stop a prescribed medicine on your own.

Long-term use of hydroquinone skin-lightening cream can turn nails brown as it darkens in light.32PubMed Central. Nail Discoloration Following the Chronic Use of Skin-Depigmenting Creams33Journal of Clinical and Investigative Dermatology. Reversible Nail Discoloration from Hydroquinone 4% Cream The stain sits on the nail surface rather than inside it,34PubMed Central. Nail Discoloration Following the Chronic Use of Skin-Depigmenting Creams and it often affects the nails of the hand used to apply the cream.35PubMed. Discolouration of the fingernails from using hydroquinone skin-lightening cream

Tiny Dark Lines and Blue Spots

Thin dark-red to black lines like splinters come from small leaking blood vessels under the nail.36PubMed Central. Nail as a window of systemic diseases A single line on one nail after a knock or pinch is common and usually means a tiny vessel bled. Thin, lengthwise, red-brown lines are more likely to be blood than pigment, and, like other blood under the nail they move out as the nail grows instead of staying fixed at the cuticle.

Splinter lines can also come from no clear cause, medicines, or skin conditions like psoriasis.37PubMed. Splinter hemorrhages of the nails: a systematic review of clinical features and associated conditions But on several nails at once they can point to body-wide illness such as a heart valve infection.38PubMed Central. Nail as a window of systemic diseases Lines across multiple nails with fever, chills, tiredness, or a new heart murmur need urgent medical care. Lines on many nails without other symptoms are still worth mentioning to a doctor, who can look for medicines or conditions as the cause.

A bluish spot on a single nail is most often a small harmless tumor, sometimes a blue mole, and rarely melanoma.39PubMed. Blue Nail Discoloration: Literature Review and Diagnostic Algorithms This tumor, called a glomus tumor, often causes sharp pain with cold or pressure, and the spot may be tiny and bluish-red. It is treated by removing it, which needs a doctor.

When to See a Doctor and What Happens There

Trapped blood can cause intense pressure pain.40PubMed. Controlled nail trephination for subungual hematoma A doctor can drill or melt a tiny hole in the nail to drain it, often relieving pain within hours.41PubMed. Controlled nail trephination for subungual hematoma It takes minutes and is usually not very painful, because the nail plate itself has no nerves. Draining helps most soon after the injury. Don’t pierce it yourself with a hot pin, which risks infection and burns. A crushed fingertip with a cut, deformity, or loose nail should be seen in case of fracture. The same goes for a bruise covering much of the nail or a torn nail edge, which may hide a fracture or nail bed cut that needs repair.

For streaks and marks, dermatologists use a lighted magnifier that helps tell blood or fungus from pigment.42PubMed Central. Use of Nail Dermoscopy in the Management of Melanonychia: Review They may photograph the nail and compare months later for width or color changes.43PubMed Central. When all you have is a dermatosoxel— start looking at the nails If signs remain suspicious, they take a biopsy from the nail root.44PubMed Central. When all you have is a dermatosoxel— start looking at the nails The area is numbed with an injection, often at the base of the finger or toe, and a small piece of the root or nail bed goes to the lab. Root biopsies can leave a permanently split or misshapen nail, especially for wide bands,45Cosmetic Dermatology. Matrix Biopsy of Longitudinal Melanonychia and Longitudinal Erythronychia: A Step-by-Step Approach so doctors weigh that risk against the risk of missing a cancer. A biopsy is usually reserved for streaks with worrying features or ones that have changed.

A new streak on one nail in an adult deserves a booking soon. A bruise with a known injury that moves outward can be watched. A mark that hasn’t moved away from the cuticle after a month or two should be checked.

To prepare, note when you first saw the mark and any injury. Bring photos showing how it has changed, and list the medicines and creams you use. Remove polish beforehand so the doctor can see the whole nail.