Podiatrist’s Answer
Most black toenails are subungual hematomas — blood pooling under the nail after trauma — and resolve on their own in 6–9 months as the nail grows out. You should see a podiatrist if: the nail is black without any trauma history, the discoloration is irregular-edged or multi-toned, or it hasn’t grown out after 9 months. These features warrant a biopsy to rule out subungual melanoma.
— Dr. Tom Biernacki, DPM, Board-Certified Podiatric Surgeon, Balance Foot & Ankle
Medically reviewed by Dr. Tom Biernacki, DPM
Board-certified podiatric surgeon | Balance Foot & Ankle
Last reviewed: May 2026
Quick answer: A black toenail has four main causes: subungual hematoma (blood pooling under the nail from trauma — by far the most common), onychomycosis (dark/black fungal nail infection variant), subungual melanoma (melanoma under the nail — rare but life-threatening — see our guide to foot and toe cancer), and Pseudomonas bacterial infection (produces a greenish-black pigment). Trauma-related black toenails resolve as the nail grows out or falls off. Subungual melanoma requires urgent biopsy. The key red flag: a dark streak or band that does not have a history of trauma and does not move with nail growth.
A black toenail is alarming to most patients — understandably so. The range of causes runs from the completely benign (blood pooled under the nail from a stubbed toe that will grow out over the next few months) to the genuinely serious (subungual melanoma, which carries the same mortality risk as any other melanoma and is often diagnosed late because people assume it must be a bruise). Getting this distinction right matters enormously. Here’s exactly how we work through it in our Howell and Bloomfield Township clinics.
Cause 1: Subungual Hematoma — Blood Under the Nail
A subungual hematoma is a collection of blood between the nail plate and nail bed, caused by trauma that ruptures the small blood vessels in the nail bed. It is the most common cause of a black or dark-colored toenail by a wide margin — in our clinic, approximately 80–85% of black toenails we see have a traumatic hematoma as the cause.
The blood appears very dark — nearly black — because it is trapped between the opaque nail plate above and the nail bed below, and because deoxygenated blood in a small dark space appears much darker than fresh bleeding. The color is a spectrum from dark red to purple to nearly black depending on the age and extent of the hematoma.
Key takeaway: Subungual hematoma — blood trapped under the nail from trauma — is the cause in the vast majority of black toenails we see in practice. The blood appears black or very dark purple under the nail plate. Critical distinguishing feature: clear trauma history, and the dark area moves forward as the nail grows over weeks to months. Without a clear trauma history, every black toenail needs clinical evaluation to rule out melanoma.
Acute large subungual hematoma: A significant crush injury or stubbed toe can produce a rapidly expanding hematoma under the nail within minutes. The buildup of blood in a confined space creates significant pressure and throbbing pain. Treatment: nail trephination — a hot wire cautery device or a sterile needle is used to make a small hole in the nail plate, immediately releasing the blood and pressure. Pain relief is dramatic and instant. This procedure is painless because the pressure is anesthetic. We perform this in our clinic regularly.
Small or old subungual hematoma: Minor trauma (a dropped object, running toe-box repetitive friction) creates a smaller hematoma that may not be immediately painful. Patients often discover the dark nail weeks after the inciting event. No treatment is needed — the hematoma resolves as the nail grows forward, eventually trimming it away over 6–12 months (toenails grow slowly). The key: monitor that the dark area moves forward with the nail over time. That is easier to judge from a dated photo than from memory: same angle, same lighting, something in frame for scale. Six weeks of nail growth is enough to show movement, and having the two pictures side by side turns a guess into a measurement. Book a black toenail evaluation online.
Key takeaway: Runner’s toenail (black toenail from repetitive toe-box trauma) affects the longest toe, which typically hits the front of the shoe on downhills and during deceleration. It is caused by a shoe that is too short OR by a shoe that fits correctly but is laced too loosely, allowing forward foot slide. The fix: ensure proper length (thumbnail’s width between longest toe and shoe end) AND secure lacing.
Cause 2: Fungal Onychomycosis — Dark Variant
Most people associate toenail fungus with yellow or white nail discoloration, but some fungal species — particularly certain non-dermatophyte molds — produce dark brown, green-brown, or nearly black nail discoloration. Scopulariopsis brevicaulis and certain Aspergillus species produce dark pigments as metabolic byproducts during their growth in the nail.
Dark onychomycosis is distinguished from subungual hematoma by the absence of trauma history, the gradual onset over many months, the presence of typical nail changes alongside the discoloration (nail thickening, crumbling, onycholysis — nail lifting from the bed), and involvement of multiple nails. KOH preparation and fungal culture confirm the diagnosis and identify the specific organism — which matters because non-dermatophyte mold onychomycosis responds differently to antifungal medications than typical dermatophyte infections.
Cause 3: Subungual Melanoma — The Critical Diagnosis
Subungual melanoma is melanoma arising from the melanocytes in the nail matrix. It accounts for 0.7–3.5% of all melanomas in Caucasians and a significantly higher proportion in people with darker skin tones — in East Asian and Black patients, subungual melanoma accounts for up to 25–35% of all melanomas. This is one of the most frequently misdiagnosed cancers because patients and physicians alike often assume a dark nail streak is a bruise.
Subungual melanoma typically presents as longitudinal melanonychia — a dark brown or black streak running lengthwise along the nail from the matrix to the free edge. As it progresses, the streak widens, becomes irregular in color and border, and eventually the most alarming sign appears: Hutchinson’s sign — extension of the pigmentation onto the periungual skin (the skin around the nail). This finding is highly specific for subungual melanoma.
Advanced subungual melanoma may cause nail destruction, nodular growth through the nail plate, and ulceration. By this stage, metastasis is common and prognosis is poor. Early diagnosis is everything.
Key takeaway: The ABCDE-F rule applies to pigmented nail bands (melanonychia): Asymmetric pigment distribution, Blurred or irregular Borders, variation in Color within the band, Diameter greater than 3mm, Extension of pigment onto the nail fold skin (Hutchinson’s sign), and the F stands for ‘Frightening’ rapid change. Any of these features in a dark nail band warrants urgent biopsy.
Diagnosis requires nail matrix biopsy. There is no imaging or clinical finding that definitively excludes melanoma — any pigmented nail lesion without a clear trauma history that meets any ABCDE-F criteria requires biopsy. We perform or coordinate nail matrix biopsies at our clinic and have a low threshold for referral to dermatology for atypical pigmentation. A benign biopsy is reassuring and inexpensive compared to a missed melanoma.
Treatment for confirmed subungual melanoma follows oncologic principles: wide local excision, sentinel lymph node biopsy for appropriate staging, and systemic therapy (immunotherapy, targeted therapy) for advanced disease, coordinated with oncology and dermatology. The 5-year survival rate for early-stage subungual melanoma approaches 80%; for distant-metastasis stage, it drops dramatically. Early diagnosis saves lives.
Cause 4: Pseudomonas Bacterial Infection
Pseudomonas aeruginosa colonization under or around the nail produces a distinctive green-black discoloration — the bacteria produces pyocyanin (blue-green pigment) and pyoverdine (green-yellow), which combine to produce a green-black appearance. This is called green nail syndrome (chromonychia).
Pseudomonas nail infection characteristically occurs when water is trapped under a partially lifted nail (onycholysis) — common in people who keep hands wet for prolonged periods (dishwashers, healthcare workers, nail technicians) or in feet with prior nail trauma or fungal infection that created nail separation. The color is distinctive — more green than black — and there is often a soft, moist, slightly malodorous tissue under the nail.
Treatment: reduce nail plate separation by cutting the nail back to the point of attachment, keep the area dry, and apply topical antibiotic (ciprofloxacin solution, dilute acetic acid soaks). Oral antipseudomonal antibiotics are reserved for severe or spreading infection.
⚠️ Black toenail features that require prompt podiatric evaluation
- No clear trauma history to explain the darkness — melanoma must be ruled out
- Dark streak or band running lengthwise that is getting wider, darker, or more irregular in border
- Pigment extending onto the nail fold skin (proximal or lateral) — Hutchinson’s sign, high concern for melanoma
- Dark area that is NOT moving forward with nail growth over 2–3 months
- Multiple different colors within the dark area (black, brown, tan all present) — atypical pigmentation pattern
- Diabetic patient with any unexplained dark nail — higher risk of infection complications
How We Distinguish Each Cause
In clinical practice, three questions resolve the diagnosis in most cases: (1) Was there a clear trauma event? (2) Is the dark area moving forward over weeks to months? (3) Does the nail have Hutchinson’s sign or any ABCDE-F features?
A dermoscopy exam (handheld magnification with polarized light) allows detailed evaluation of pigment pattern, helping distinguish blood (irregular red-brown globules that smear) from melanocytic pigment (parallel ridge pattern on nail bed, brown-gray color). For any case where trauma history is unclear or dermoscopy findings are atypical, nail biopsy is the only definitive answer.
Black Toenail Causes: How to Tell Them Apart
| Cause | Typical look | The deciding clue | Urgency |
|---|---|---|---|
| Subungual hematoma (trauma) — ~80–85% of cases | Dark red → purple → near-black under the nail | Clear injury; the dark area grows out with the nail over months | Routine; drain only if painful/throbbing |
| Dark fungal infection (onychomycosis) | Brown / green-brown to black, with thickening & crumbling | No trauma; usually multiple nails, gradual over months | See a podiatrist; confirm by KOH/culture |
| Subungual melanoma | Brown/black lengthwise streak, often irregular or widening | No trauma; pigment onto the skin (Hutchinson’s sign) or any ABCDE-F feature | Urgent — biopsy to rule out melanoma |
| Pseudomonas (“green nail”) | Distinctive green-black | Often under a lifted nail; moist, faintly malodorous | Treatable; keep dry + topical; see podiatrist if spreading |
Frequently Asked Questions
Should I be worried about a black toenail? If you clearly stubbed or dropped something on the toe and the blackness developed within hours of the injury, it’s almost certainly a hematoma and not concerning. If there was no injury, if the dark area is a lengthwise streak (not a uniform dark area), if it’s getting darker or wider, or if you’ve had it for months and it doesn’t seem to be growing forward — get it evaluated. Melanoma is rare but real, and early detection is the entire difference in outcomes.
How long does a black toenail from running take to heal? Runner’s subungual hematoma from shoe friction typically takes 3–6 months for the dark area to grow forward and be trimmed away. If the hematoma is large enough, the nail may detach from the bed and fall off — which sounds alarming but is generally painless and the new nail grows back normally beneath. Address the shoe fit to prevent recurrence: proper length plus secure lacing.
Can I drain a black toenail at home? A painful, throbbing black toenail from a fresh injury can be drained via nail trephination for immediate pain relief — but this should ideally be done in a medical office with sterile equipment. Home attempts with a needle risk infection and incomplete drainage. If it’s been more than 48–72 hours since the injury and the pain has subsided, drainage is no longer necessary — the pressure has dissipated.
The Bottom Line
A black toenail with a clear trauma history that moves forward with nail growth is almost certainly benign. A dark nail streak without trauma history, especially with any ABCDE-F features, is melanoma until proven otherwise by biopsy. The vast majority of black toenails we evaluate are hematomas or fungal — but we never skip the clinical evaluation because the consequences of missing a subungual melanoma are too serious. If you’re uncertain, a single office visit resolves the question definitively.
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American Academy of Dermatology: Nail Problems
Black Toenails in Runners and Hikers
The single most common real-world cause of a black toenail is not one dramatic injury. It is thousands of small ones. Every stride drives the toe forward against the inside of the shoe, and over a long run or a long descent that repetition ruptures the small vessels under the nail. The bleeding collects, and the nail goes dark days later — often with no memory of anything having happened.
This is why the classic presentation is a runner who finishes a half marathon feeling fine and notices a black second toenail the following Tuesday.
Why downhill is worse than uphill
Running or hiking downhill is the biggest single risk factor, and it catches people out because it feels easier than climbing. On a descent, gravity slides the whole foot forward in the shoe on every step, so the toes meet the front of the toe box with far more force than on the flat. Long descents — a trail race, a mountain hike, the back half of a hilly marathon — do most of the damage.
Getting the shoe right
Most runner’s black toenails are a fit problem, and fit is fixable.
- Go up about half a size from your street shoe. Running feet are not walking feet. You need roughly a thumb’s width between the longest toe and the front of the shoe.
- Measure at the end of the day, or straight after a run. Feet swell measurably over a long effort; a shoe fitted on a cold morning can be too short at mile 18.
- Check toe box height, not just length. A shallow toe box abrades the top of the nail even when the shoe is long enough. If you have a raised second toe or a hammertoe, height matters more than length.
- Know which toe is longest. Plenty of people have a second toe longer than the big toe, and it is the one that hits first. Fit the shoe to the longest toe you actually have.
The lacing fix most runners have never been shown
If the shoe is the right length and your toes still hit the front, the problem is the foot sliding forward — and that is a heel-hold problem, not a length problem. A heel lock, sometimes called a runner’s loop, solves it: use the extra eyelet at the top of the shoe to make a loop on each side, thread the opposite lace through, and tighten. It anchors the heel back into the heel counter so the foot stops migrating forward on descents.
It costs nothing, takes one attempt to learn, and resolves a surprising share of recurrent black toenails on its own.
The rest of the prevention list
- Trim nails straight across and short before a long effort — but not into the corners. A long nail meets the shoe first; a nail cut down the sides invites an ingrown one instead.
- Wear properly fitted technical socks. Bunching adds bulk in exactly the wrong place, and damp cotton increases friction.
- Lace tighter for descents, looser for climbs.
- Replace worn shoes. A collapsed upper stops holding the foot regardless of how it is laced.
When a runner’s black toenail is not just trauma
It is easy to attribute every dark nail to training, and that assumption is the reason the serious diagnosis gets missed. Take a second look if:
- Only one nail is affected and there was no long run, race or hike to explain it.
- The dark area does not grow out with the nail over the following months. Trauma moves forward as the nail grows; pigment produced by the nail matrix does not.
- Pigment extends onto the surrounding skin at the base or the side of the nail.
- The band is widening, or is wider at the cuticle than at the tip.
Those features point toward subungual melanoma rather than a bruise, and they are covered in more detail in the melanoma section above. Runners are not exempt — if anything, they are at higher risk of a delayed diagnosis precisely because a black toenail seems self-explanatory.
If a dark nail is painful, is not growing out, or is the only one of its kind, it is worth having examined. Balance Foot & Ankle sees patients in Howell (4330 E Grand River Ave, Howell, MI 48843) and Bloomfield (43494 Woodward Ave #208, Bloomfield Township, MI 48302). Call (810) 206-1402.
Sources
- Braun RP, Baran R, Le Gal FA, et al. Diagnosis and management of nail pigmentations. J Am Acad Dermatol. 2007;56(5):835-847.
- Haneke E, Baran R. Longitudinal melanonychia. Dermatol Surg. 2001;27(6):580-584.
- Levit EK, Kagen MH, Scher RK, et al. The ABC rule for clinical detection of subungual melanoma. J Am Acad Dermatol. 2000;42(2 Pt 1):269-274.
- Piraccini BM, Antonucci A, Rech G, et al. Longitudinal melanonychia due to Hailey-Hailey disease. Eur J Dermatol. 2006;16(1):62-63.
- Thai KE, Young R, Sinclair RD. Nail apparatus melanoma. Australas J Dermatol. 2001;42(2):71-83.
Related guide: Black or discolored toenails are often related to nail fungus. See Our Toenail Fungus Treatment Guide — laser, antifungals, and what actually clears onychomycosis long-term.
What causes a black toenail?
Black toenails (subungual hematoma) result from blood pooling under the nail from direct trauma — dropping something on the foot, or repetitive pressure from shoes during running. Rarely, unexplained black nail discoloration can indicate subungual melanoma, which a podiatrist can differentiate from trauma-related hematoma on examination.
Should I drain a black toenail?
Small hematomas (covering less than 25% of the nail) can be left alone to resolve as the nail grows out. Large, painful hematomas (>50% of nail) can be drained with a quick office procedure using a heated needle to relieve pressure and prevent nail loss. Do not attempt to drain it at home — infection risk is significant.
Will a black toenail fall off?
Yes — large subungual hematomas often cause the nail to detach as new nail grows underneath. This process takes 6–9 months. The new nail grows in normally in most cases. See a podiatrist if the nail is causing significant pain, shows signs of infection, or if the discoloration is not clearly related to trauma.
What causes a black toenail?
The three most common causes are: (1) subungual hematoma — blood pooling under the nail from trauma or repetitive pressure in running shoes; (2) toenail fungus (onychomycosis) — usually with thickening and crumbling; and (3) subungual melanoma — rare but serious, presenting as a dark streak or pigment that extends onto the skin (Hutchinson’s sign).
When is a black toenail a sign of something serious?
A black toenail needs urgent evaluation if: the pigment appears as a streak (melanonychia) running the length of the nail, the dark area extends onto the surrounding skin, there was no trauma, the discoloration is worsening over weeks, or you have a history of melanoma. Subungual melanoma accounts for ~5% of all melanomas and is frequently diagnosed late.
Does a black toenail from trauma need to be removed?
Only if the hematoma involves more than 25–50% of the nail and is very painful. In that case, a podiatrist can perform nail trephination — a small hole drilled to release the pressure — which provides immediate relief. Most traumatic black toenails resolve on their own in 6–9 months as the new nail grows in, requiring no treatment beyond proper shoe fit.
Related reading: toenail fungus · toenail ridges · broken toe
In-Office Treatment at Balance Foot & Ankle
Dr. Tom Biernacki DPM provides expert in-office care at Balance Foot & Ankle, serving Howell and Bloomfield Township, Michigan. Learn more about laser toenail treatment in Michigan. Same-day appointments: (810) 206-1402 | New Patient Information
Specialist For This Condition
Dr. Daria Gutkin, DPM, AACFAS is the Balance Foot & Ankle podiatrist most patients ask for when a discolored or injured toenail needs a gentle, thorough evaluation. Cleveland Clinic-trained, bilingual (English / Russian), known as the gentlest hands in Michigan podiatry. Call (810) 206-1402 to request Dr. Gutkin at the Howell or Bloomfield Township office.
Related dark-nail guides
Sorting trauma from something more serious? Compare these:
Concerned about a black or discolored toenail? See when to seek care in our foot & ankle conditions guide.
Dr. Tom Biernacki, DPM is a board-certified foot & ankle surgeon (ABFAS & ABPM) at Balance Foot & Ankle Specialists in Southeast Michigan. With over a decade of clinical experience, he specializes in heel pain, bunions, diabetic foot care, sports injuries, and minimally invasive surgery. Dr. Biernacki is a member of the APMA and ACFAS, and his patient education content on MichiganFootDoctors.com and YouTube has made him one of the most-followed foot & ankle educators on YouTube.
More questions patients ask
Will a black toenail heal on its own?
Most black toenails from injury heal on their own as the nail grows out over 6–12 months. If very painful, a podiatrist can drain the blood. See a doctor if the nail changes are not related to trauma or don't grow out normally.
When should I see a doctor for a black toenail?
See a podiatrist if your black toenail is very painful, not related to any injury, has a streak pattern, or doesn't grow out normally. These could signal fungal infection or, rarely, melanoma beneath the nail.
Can a black toenail be a sign of cancer?
Yes, though it is rare. Subungual melanoma accounts for about 0.7–3.5% of all melanoma cases and is more common in people with darker skin tones. Warning signs of melanoma vs. simple bruising: the dark streak extends into the cuticle (Hutchinson's sign), the discoloration is a streak rather than a blotchy bruise, it does not grow out with the nail over 3–4 months, and there is no history of trauma. Any black toenail without a clear trauma history should be evaluated.
How long does it take for a black toenail to grow out?
A black toenail from subungual hematoma (bruising) typically resolves as the nail grows out over 3–6 months for fingernails and 6–12 months for toenails. If the nail is lifted by significant blood pooling, it may fall off — the new nail grows in underneath and the process takes 6–9 months total. If the discoloration is growing rather than growing out toward the tip, this is a red flag and warrants evaluation.
When should I see a podiatrist for a black toenail?
See a podiatrist if: there was no obvious trauma, the discoloration has not shifted toward the nail tip after 2–3 months, the dark area is a vertical streak extending to the cuticle, you notice skin changes around the nail fold, you have significant pain or pressure, or you are diabetic (black toenails can mask nail infections in diabetic patients). Early evaluation is always safer than waiting.
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