A black toenail is almost always caused by trauma — a single hard impact or repeated pressure that ruptures tiny blood vessels under the nail, creating a pool of blood called a subungual hematoma that appears dark purple, blue, or black.
Less commonly, black discoloration comes from fungal infection, melanonychia (increased pigment in the nail bed), or — rarely — subungual melanoma. Most black toenails from trauma heal on their own in 6 to 9 months as the nail grows out, but certain warning signs require prompt medical evaluation to rule out serious conditions.
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The black color you see is almost always blood trapped beneath the nail plate — when blood vessels rupture and have nowhere to drain, the pooled blood oxidizes and darkens over hours to days.
Your toenail sits on a nail bed packed with capillaries. Any force that crushes these vessels causes bleeding into the confined space between the nail plate and nail bed. Because the blood cannot escape, it spreads beneath the nail and clots. Fresh blood appears red or dark purple. As hemoglobin breaks down over 24 to 48 hours, the color shifts to blue, then black or dark brown.
The nail plate itself is translucent keratin. It acts like a window showing whatever sits beneath it. A hematoma (blood collection) looks alarming but is simply blood showing through that window.
Yes — acute trauma is the single most common cause of sudden black toenails.
Dropping a heavy object, stubbing your toe hard, or slamming it in a door crushes the nail bed instantly. Blood pools within minutes. You'll likely remember the exact moment it happened because it hurts — a lot. The nail often throbs for hours afterward as pressure builds in the confined space.
"Subungual hematomas result from direct trauma to the nail plate, which causes bleeding from the nail matrix into the space beneath the nail." — American Academy of Family Physicians
Repeated microtrauma from athletic activity is the second most common cause — runners call this "runner's toe" or "tennis toe."
Every time your foot slides forward in your shoe and your toenail hits the inside of the toe box, you create tiny amounts of damage. One impact is nothing. Ten thousand impacts over a marathon or a multi-day hike accumulate into a full hematoma. You may not notice pain during the activity, only discovering the black nail when you remove your shoes.
Shoes that are too small, too loose, or worn without proper socks increase risk. Downhill running is particularly brutal because your toes jam forward with each stride. A 2019 study in the British Journal of Sports Medicine found that up to 14% of marathon runners develop black toenails during training or racing.
Fungal nail infection (onychomycosis) can cause dark brown or black discoloration, but it usually starts as yellow or white and progresses over months, not hours.
Certain fungal species — especially molds and some dermatophytes — produce dark pigments as they colonize the nail. The discoloration tends to be patchy or streaky rather than the uniform dark pool of a hematoma. Fungal nails also thicken, crumble at the edges, and may smell unpleasant.
If your nail turned black gradually over weeks or months without any injury you can recall, fungal infection moves higher on the list of suspects. Fungal infections don't resolve without treatment and can spread to other nails.
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Melanonychia is a longitudinal pigmented band — a vertical stripe running from the cuticle to the nail tip — caused by increased melanin production in the nail matrix.
This is more common in people with darker skin tones and is often completely benign. The band appears brown to black and grows out with the nail. However, melanonychia can also be a sign of subungual melanoma (see below), so new pigmented bands that appear in adults should be evaluated by a dermatologist.
Subungual melanoma is rare but serious — it accounts for about 0.7% to 3.5% of all melanomas and often presents as a dark streak or spot under the nail.
Unlike a hematoma, melanoma pigmentation does not grow out with the nail. It persists in the same location and may expand over weeks to months. The "ABCDEF" rule helps distinguish suspicious lesions:
| Warning Sign | What to Look For |
|---|---|
| A — Age and race | Peak incidence ages 40–70; higher risk in African Americans, Asians, and Hispanics |
| B — Band characteristics | Width >3mm, irregular borders, multiple colors |
| C — Change | Rapid increase in size or darkening |
| D — Digit involved | Thumb, big toe, or index finger most common |
| E — Extension | Pigment spreading onto the cuticle or surrounding skin (Hutchinson's sign) |
| F — Family history | Personal or family history of melanoma |
"Subungual melanoma is frequently diagnosed late because it is often initially attributed to trauma. Any pigmented lesion that does not grow out with the nail over 3–4 months warrants biopsy." — Journal of the American Academy of Dermatology
Some drugs cause nail pigmentation as a side effect — chemotherapy agents (especially doxorubicin and cyclophosphamide), antimalarials, and certain antibiotics can darken nails.
If you started a new medication in the weeks before noticing discoloration and have no history of trauma, mention the nail change to your prescribing doctor. Drug-induced pigmentation is usually reversible once the medication stops.
The timeline and pattern of discoloration are your best diagnostic clues — hematomas appear suddenly after trauma and grow out with the nail, while fungal and melanoma changes evolve slowly and stay in place.
| Feature | Subungual Hematoma | Fungal Infection | Subungual Melanoma |
|---|---|---|---|
| Onset | Sudden, often after known injury | Gradual over weeks/months | Gradual, no injury |
| Location | Pool of color, often under center of nail | Patchy, may start at edges | Longitudinal band or spot |
| Color | Red → purple → black → brown | Yellow, white, brown, black streaks | Brown to black, may have multiple hues |
| Movement | Grows out toward tip over months | Stays in place or spreads | Stays in place or enlarges |
| Nail texture | Normal unless lifting | Thick, crumbly, brittle | May be normal initially |
| Pain | Often painful initially | Usually painless | Usually painless |
| Surrounding skin | Normal | Normal or mildly inflamed | May show pigment spread (Hutchinson's sign) |
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Most traumatic black toenails need no treatment — the hematoma grows out as the nail grows, typically over 6 to 9 months for a big toenail.
If you caught the injury within the first 24 to 48 hours and the nail is throbbing painfully from pressure, a doctor can perform trephination — creating a small hole in the nail plate to release the trapped blood and relieve pressure immediately. Do not attempt this at home unless you have medical training and sterile equipment. Infection risk is real.
For at-home care:
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Seek medical evaluation if the discoloration has no clear cause, does not grow out after 3 to 4 months, spreads beyond the nail, or is accompanied by signs of infection.
Red flags that warrant a visit:
A podiatrist or dermatologist can examine the nail with a dermatoscope and, if needed, take a biopsy to rule out melanoma or confirm fungal infection with a culture or KOH preparation.
Proper footwear fit is the most effective prevention — your running shoes should have a thumb's width of space between your longest toe and the end of the shoe.
Other preventive measures:
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A black toenail is usually a subungual hematoma from trauma — blood trapped under the nail after a direct hit or repetitive pressure from sports. It looks alarming but typically heals on its own as the nail grows out over several months. Fungal infections cause slower, patchier discoloration and require antifungal treatment. The rare but serious concern is subungual melanoma — any dark band that doesn't grow out, spreads to surrounding skin, or changes in appearance should be evaluated by a dermatologist. When in doubt, get it checked.
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Sometimes. If the hematoma is large and lifts the nail plate from the nail bed, the nail may loosen and eventually detach completely. This is painless by the time it happens. A new nail is already growing underneath. Avoid pulling the nail off prematurely — let it detach naturally to protect the new nail bed.
Toenails grow about 1.5 to 2mm per month. A big toenail takes 6 to 9 months to grow out completely. Smaller toenails grow faster. The black discoloration will gradually move toward the tip and eventually be trimmed away as part of normal nail maintenance.
Yes, especially if the nail is lifting or has an open wound beneath it. Watch for increasing redness, warmth, swelling, pus, or red streaks traveling up the foot. These are signs of bacterial infection requiring antibiotics. People with diabetes or poor circulation should have any nail changes evaluated promptly.
No. While trephination (drilling or burning a hole through the nail to release blood) provides rapid pain relief, it must be done with sterile technique to avoid introducing infection. A healthcare provider can perform this safely within 48 hours of injury. After 48 hours, the blood has clotted and drainage is no longer effective.
No. The vast majority of black toenails result from trauma and are nothing more than a bruise under the nail. However, black discoloration that appears without injury or does not grow out over several months should be evaluated to rule out melanoma or other nail bed pathology.
Reviewed and Updated on April 17, 2026 by George Wright