Most black big toenails are bruises: blood trapped beneath the nail after the toe is hit or squeezed. The squeeze doesn’t need to be dramatic. Months of a toe tapping the front of a shoe can do the same thing as a dropped weight.1Dr. Dana Stern Nail Dermatologist. Nail Trauma and Nail Bed Injury from Sports or Nailcare
A few rarer causes, including a form of skin cancer, can look similar. So what matters most is not the color itself but how the darkness behaves over the following weeks.
A Bruise Under the Nail Is the Usual Cause
Small blood vessels in the nail bed break open, and blood pools between the bed and the hard nail.2Banner Health. Why Is My Big Toenail Black? Causes and When to Worry3Banner Health. Why Is My Big Toenail Black? Causes and When to Worry The nail can then look black, blue or purple.4Banner Health. Why Is My Big Toenail Black? Causes and When to Worry
There are two ways it happens. One is a single blow, such as dropping something heavy on the foot or slamming a toe.5Banner Health. Why Is My Big Toenail Black? Causes and When to Worry A crush like that can cause intense, throbbing pain as blood collects with nowhere to go.6Manchester Podiatry. Subungual haematomas (Bleeding under nail) The other is repeated rubbing or friction against a shoe.7Banner Health. Why Is My Big Toenail Black? Causes and When to Worry That repeated microtrauma comes from running, hiking, skiing, tennis, dancing or tight shoes.8Dr. Dana Stern Nail Dermatologist. Nail Trauma and Nail Bed Injury from Sports or Nailcare With this kind, you may not remember any injury at all, even though the cause is still a bruise.
Downhill walking and running push the foot forward, so the nail strikes the toe box with every step. A shoe half a size too small, or laced loosely, lets the foot slide and bang the nail. Thick or long nails take more of the force than short, trimmed ones.
A fresh bruise often looks red-purple and may throb for the first day or two. Over the following days the trapped blood dries and darkens to deep maroon or black. An old runner’s bruise is often painless and gets noticed only when trimming nails. The dark patch is usually a blob with a fairly even color, rather than a thin straight stripe.
Caring for a Bruised Toenail
A small, painless bruise usually needs no treatment and grows out with the nail. Toenails grow slowly, roughly 1 to 2 mm a month, so a mark near the base can take many months to reach the tip. Rest, cool packs, raising the foot and roomy shoes ease the first days.
When throbbing is severe, a clinician can make a small hole in the nail to let the blood out.9PubMed. Controlled nail trephination for subungual hematoma Done with the right tool, it penetrates only the nail, not the tissue beneath, and carries little risk.10PubMed. Controlled nail trephination for subungual hematoma People with bleeding disorders such as haemophilia often need the nail drained or removed, so they should get care early.11PubMed. Hand and Foot Subungual Haematomas in Haemophilia: A Report of Six Cases Don’t burn or drill the nail yourself, since a home attempt risks infection and damage to the nail bed.
Draining works best soon after the injury, while the blood is still liquid. A bruise covering most of the nail after a crush can mean the bone or nail bed is also hurt. A cut around the nail, swelling or a bent toe needs an exam for a fracture. People with diabetes or poor circulation should have any toe injury checked.
A large bruise can lift the nail so it loosens and comes off over weeks. Leave a loosening nail in place and keep it covered, because it protects the bed underneath. A new nail grows in behind it, which for a big toe can take up to a year or more. Trim jagged edges so they don’t catch on socks.
Fungal and Bacterial Infections That Darken Nails
Nail fungus usually turns nails yellow or white, but some fungi make melanin, the same dark pigment found in skin.12Journal of the American Academy of Dermatology. Why Is My Big Toenail Black? Causes and When to Worry These can make the nail look brown or black.13Journal of the American Academy of Dermatology. Why Is My Big Toenail Black? Causes and When to Worry They live in soil, plants and decaying debris, and at least 21 kinds have been found in blackened nails.14Journal of the American Academy of Dermatology. Why Is My Big Toenail Black? Causes and When to Worry Gardeners, farm workers and people often barefoot outdoors are more exposed.15PubMed Central. Onychomycosis caused by dematiaceous fungi: A four-year study on agricultural workers of Assam, India
A common bacterium can stain the nail greenish-yellow, greenish-brown or greenish-black, a condition sometimes called green nail syndrome.16PubMed Central. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons The bacterium makes a blue-green pigment that tints the nail.17PubMed Central. Green Nail Syndrome Treated with the Application of Tobramycin Eye Drop The nail often lifts at the tip, and the skin fold at the base can be puffy.18PubMed Central. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons It is more likely when nails stay wet, are exposed to soaps and detergents, or have been injured, especially in older adults.19PubMed Central. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons
Infection looks different from a bruise. The nail may be thickened, crumbly or lifting from the bed, not just discolored. Darkness often starts at the tip or sides rather than in a single patch. A greenish tint, especially under a lifted edge, suggests the bacterial cause. More than one affected nail, or athlete’s foot on nearby skin, makes fungus likelier.
A clinician can clip a piece of nail to test for fungus, because the pigment-making types need the right treatment. Over-the-counter creams rarely clear nail fungus well, though prescription options exist. Keeping feet dry and changing damp socks helps with both infections. Assuming dark discoloration is fungus and self-treating for months without a check is a mistake.
Medicines, Circulation and Harmless Pigment Streaks
The nail root contains pigment cells that normally stay quiet.20PubMed Central. Narrative Review of Drug-Associated Nail Toxicities in Oncologic Patients When they become active, they can lay down a dark stripe running from base to tip, which may be harmless or may come from a mole in the nail root.21PubMed. Evaluation and diagnosis of longitudinal melanonychia: A clinical review by a nail expert group Activation can come from normal variation, local irritation, body-wide conditions or medicines.22PubMed. Evaluation and diagnosis of longitudinal melanonychia: A clinical review by a nail expert group Darker-skinned people more often have harmless brown nail stripes, often on several nails.
Darkened nails are one of the most common nail side effects of chemotherapy.23PubMed Central. Narrative Review of Drug-Associated Nail Toxicities in Oncologic Patients The color usually shows up 1 to 2 months into treatment.24PubMed Central. Narrative Review of Drug-Associated Nail Toxicities in Oncologic Patients It typically affects several nails at once.25PubMed Central. Narrative Review of Drug-Associated Nail Toxicities in Oncologic Patients
Poor blood flow to the feet starves nails of oxygen and nutrients.26USA Vascular Centers. Peripheral Artery Disease & Toenails Nails can grow brittle, thick or discolored as a result.27USA Vascular Centers. Peripheral Artery Disease & Toenails
How many nails are affected is a useful clue. Medicines and body-wide causes tend to affect several nails, often fingernails too, so mention any new medicines to the doctor. A change in just one nail more often points to a local cause: injury, infection or a growth. A single new stripe in an adult still deserves a check, even though harmless causes are common.
Cold feet, shiny hairless skin on the shins and leg pain when walking can signal poor circulation. Smokers and people with diabetes are at higher risk. Slow-healing sores on the toes are more urgent than the nail color. Truly black, dry skin on a toe, as opposed to the nail, needs urgent care.
Could a Black Toenail Be Melanoma
Melanoma can grow under the nail, and the big toe and thumb are common spots. Bruises, fungus, harmless moles and drug-related color all look similar, so doctors must rule them out.28PubMed Central. Subungual Melanoma is Not so Rare: Report of Four Cases from India Sports bruises under the nail are common enough to be confused with this cancer.29Quality in Sport. Mechanical and Friction-Induced Dermatoses in Athletes: Black Heel, Piezogenic Papules, and Related Traumatic Skin Conditions – a narrative review
Concerning stripes tend to be uneven, with lines of different thickness, spacing and color.30PubMed Central. When all you have is a dermatoscope— start looking at the nails Asymmetry, fading blurry edges, several colors and a band at least 3 mm wide were each linked to early nail melanoma.31PubMed Central. Assessment of a Predictive Scoring Model for Dermoscopy of Subungual Melanoma In Situ Dark color spreading onto the skin around the nail is a key warning sign.32Actas Dermo-Sifiliográficas. Is Hutchinson’s Sign Pathognomic of Subungual Melanoma? Dark pigment seen through a thin cuticle, or a harmless mole in children, can mimic that sign.33Actas Dermo-Sifiliográficas. Is Hutchinson’s Sign Pathognomic of Subungual Melanoma?
You can do some checking at home. Mark the top edge of the dark area with a fine scratch or a photo, then look again in two to four weeks. Bruise color moves forward toward the tip as the nail grows, and clear nail appears behind it, while melanoma stays rooted at the base. A bruise that grows out doesn’t rule out a problem underneath if new color keeps appearing at the base. A stripe that gets wider or darker over months is a reason to book an appointment.
Some people should take a single dark nail more seriously: adults over 50 with a new streak and no clear injury, people with a personal or family history of melanoma, and anyone whose nail splits, bleeds or lifts around a dark band. The cancer is very rare in children, so a child’s dark stripe is usually harmless, though a changing one should still be checked.
When to See a Doctor and What the Visit Involves
Book an appointment if the dark color appeared with no injury you can remember, or if the mark hasn’t moved forward after about a month. The same goes for a stripe from base to tip, especially a new one in an adult, and for color on the skin around the nail, bleeding, or a split nail. Pus, spreading redness or fever suggest infection.
The visit starts with questions about injuries, sports, shoes, medicines and how long the color has been there. Your dated photos are very helpful, so bring them. A dermatologist often looks with a lighted handheld magnifier.34Journal of Dermatology for Physician Assistants. Overview of onychoscopy: Dermoscopy for the nail It helps tell pigment from pigment cells apart from blood, fungus and other causes.35Journal of Dermatology for Physician Assistants. Overview of onychoscopy: Dermoscopy for the nail It can also spot the irregular line patterns and uneven colors of early melanoma.36Journal of Dermatology for Physician Assistants. Overview of onychoscopy: Dermoscopy for the nail That can avoid unnecessary surgery and guide which biopsy, if any, is needed.37Elsevier / PubMed Central. Diagnosis and management of nail pigmentations Depending on what the doctor sees, you may have a nail clipping tested for fungus, or a small biopsy from the nail root done under local numbing.
Simple letter checklists for nail stripes did not reliably separate melanoma from harmless stripes in one clinic’s patients.38PubMed. Retrospective single-center study evaluating clinical and dermoscopic features of longitudinal melanonychia, ABCDEF criteria, and risk of malignancy So leave the final call to an exam rather than counting signs yourself.
Preventing Black Toenails From Shoes and Sport
Repeated injury to the nail root can permanently leave nails thickened, lifted, painful or growing abnormally.39Dr. Dana Stern Nail Dermatologist. Nail Trauma and Nail Bed Injury from Sports or Nailcare40Dr. Dana Stern Nail Dermatologist. Nail Trauma and Nail Bed Injury from Sports or Nailcare Leave about a thumb’s width between the longest toe and the shoe tip. Shop for shoes later in the day when feet are largest, and size up for long runs or hikes. A heel-lock lacing pattern stops the foot sliding forward downhill. Cut toenails straight across and short so the tip doesn’t catch the shoe. Moisture-wicking socks, swapped out when wet, cut infection risk too.
Steel-toe boots that are too short or narrow press on the big toe all shift. Try boots on with the socks you work in, and pick wide-toe-box styles if your toes feel cramped. If nail bruising keeps happening, tell your podiatrist, who can suggest inserts.
Living With a Dark Nail While It Grows Out
Nail polish can hide a confirmed bruise, but remove it before checks so changes stay visible. Don’t paint over an unexplained dark streak, which hides the very change you need to watch. Closed shoes or toe covers work while the new nail grows in. A podiatrist can tidy a damaged or lifting nail so it looks and feels better.