What Causes Toenails to Turn Black — and When to See a Doctor

Medically reviewed by Dr. Tom Biernacki, DPM — Board-Certified Podiatric Surgeon — Balance Foot & Ankle, Howell & Bloomfield Hills, MI. Last updated April 2026.

Is a Black Toenail Serious?

Discovering that one of your toenails has turned black can be alarming. Fortunately, the most common causes are benign — but some causes of black or darkened toenails require prompt medical attention, and one uncommon cause (subungual melanoma) can be life-threatening if missed. Understanding what distinguishes these causes helps you determine when to monitor at home versus when to see a podiatrist promptly.

At Balance Foot & Ankle in Howell and Bloomfield Township, Michigan, Dr. Tom Biernacki DPM and his team regularly evaluate nail discoloration and can determine the cause quickly, providing peace of mind or early intervention as appropriate.

Subungual Hematoma: The Most Common Cause

The most frequent cause of a black toenail is a subungual hematoma — blood pooled beneath the nail following injury to the nail bed. Trauma is usually obvious (dropping a heavy object on the toe, stubbing it hard, or having it stepped on), producing immediate pain followed by progressive darkening of the nail over the subsequent hours as blood collects beneath the nail plate.

In runners, the repetitive trauma of toes hitting the front of the shoe — particularly during downhill running or wearing shoes that are too small — can produce subungual hematomas without a single traumatic event. Runners frequently lose toenails during marathon training, particularly the first and second toes. The darkening in these cases develops more gradually than with acute trauma.

Small subungual hematomas occupying less than 25–50% of the nail surface in an otherwise healthy toe typically resolve on their own without intervention, with the nail eventually growing out and the discoloration disappearing. Large, painful hematomas can be drained by a podiatrist using a technique called trephination — creating a small hole through the nail to allow blood to escape — providing immediate pain relief. Any crushed or injured toe requires X-ray evaluation to rule out an underlying fracture of the distal phalanx.

Toenail Fungal Infection (Onychomycosis)

While onychomycosis (toenail fungus) most commonly produces white, yellow, or brown nail discoloration, it can occasionally cause dark brown to black nail changes, particularly when specific fungal species are involved or when secondary bacterial contamination occurs within the dystrophic nail. The associated nail changes — thickening, brittleness, crumbling, separation of nail from nail bed (onycholysis) — help distinguish fungal infection from traumatic hematoma.

Fungal nail infections do not resolve spontaneously and worsen over time. Treatment options include topical antifungal agents for mild cases, oral antifungals (most commonly terbinafine) for more extensive involvement, and laser treatment for appropriate candidates. Accurate diagnosis by nail clipping and laboratory culture or PCR testing is valuable before committing to months of oral antifungal therapy, as non-fungal nail conditions can mimic onychomycosis clinically.

Subungual Melanoma: The Diagnosis That Cannot Be Missed

Subungual melanoma — malignant melanoma arising from the nail matrix — is rare but potentially fatal if diagnosis is delayed. It accounts for approximately 1–3% of all melanomas but up to 20% of melanomas in people with darker skin pigmentation. The most common presentation is a longitudinal melanonychia — a dark brown or black streak running along the length of the nail — known as a melanonychia striata. When this stripe is caused by melanocyte activation rather than melanoma, it is a benign finding, but distinguishing benign from malignant pigmentation requires expert evaluation.

Features that raise concern for subungual melanoma include: a band of pigmentation wider than 3mm, irregular or blurred borders within the band, extension of pigment onto the surrounding skin (Hutchinson’s sign — a finding that should prompt urgent referral), multiple colors within the band, sudden appearance or rapid change in a pre-existing band, or nail dystrophy in association with pigmentation. Any of these features warrants urgent evaluation and likely nail matrix biopsy.

The ABCDEF criteria adapted for nail melanoma (A = Age 40–70; B = Band of pigmentation, dark brown/black; C = Change in band or lack of change with treatment; D = Digit involved — thumb, hallux, or index finger most common; E = Extension of pigment to surrounding skin; F = Family or personal history of melanoma or dysplastic nevi) provides a useful clinical framework for risk stratification. When in doubt, biopsy is the appropriate course of action — subungual melanoma has a much better prognosis when diagnosed at an early stage.

Other Causes of Dark Toenails

Certain medications, including antimalarials, minocycline, and some chemotherapy agents, can cause diffuse nail pigmentation. Systemic conditions including Addison’s disease and HIV can cause nail darkening. Repeated minor friction from footwear can cause pigment changes in the nail bed over time. Bacterial infection beneath the nail — particularly with Pseudomonas aeruginosa — produces a characteristic greenish-black discoloration that should prompt prompt treatment with appropriate topical or systemic antibiotics.

When to See a Podiatrist

Any black or dark toenail that appeared without clear trauma, any pigmented streak in the nail that is new, changing, or meets any of the concerning ABCDEF criteria, any nail discoloration accompanied by pain, swelling, or signs of infection, or any nail condition that persists beyond expected healing time warrants evaluation. A podiatrist can typically determine the cause of nail discoloration through examination and, when necessary, nail clipping, culture, or biopsy, providing definitive diagnosis and appropriate treatment.

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When to See a Podiatrist for Toenail Problems

A black toenail can result from trauma, fungal infection, or rarely, melanoma under the nail. Dr. Tom Biernacki at Balance Foot & Ankle can quickly determine the cause and provide appropriate treatment, including biopsy if a suspicious lesion is identified.

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Clinical References

1. Haneke E. “Ungual melanoma: controversies in diagnosis and treatment.” Dermatologic Therapy. 2012;25(6):510-524.
2. Jellinek NJ. “Nail matrix biopsy of longitudinal melanonychia.” Journal of the American Academy of Dermatology. 2007;56(5):803-810.
3. Daniel CR, et al. “The toenail: a diagnostic and therapeutic challenge.” Cutis. 1996;57(5):381-384.