Black Toenail: Trauma vs. Fungus vs. Melanoma—How to Tell the Difference

Quick answer:A bruised toenail is blood trapped under the nail after stubbing the toe or wearing tight shoes, leaving it dark and tender. It usually grows out over months. See a podiatrist if the pain is severe, the toe may be fractured, or a dark area appears with no injury. Call (810) 206-1402.

Related Conditions

Medically reviewed by Dr. Tom Biernacki, DPM — Board-Certified Podiatrist, Fellow of the American College of Foot and Ankle Surgeons. Updated April 2026.

What Causes a Black Toenail?

A black or dark toenail is one of the most common reasons patients visit a podiatrist, and getting the diagnosis right matters significantly—because the causes range from completely benign (a bruise under the nail from running) to potentially serious (subungual melanoma). The three most important causes are subungual hematoma (blood under the nail from trauma), onychomycosis (nail fungus with dark pigmentation), and melanonychia or subungual melanoma (dark pigmentation from nail cells or cancer). Each has distinct features that help distinguish them, though nail biopsy is sometimes the only way to be certain.

Subungual Hematoma: The Traumatic Black Toenail

Subungual hematoma—blood pooling between the nail plate and nail bed—is the most common cause of a dark toenail. It results from direct trauma (dropping something on the toe, stubbing the toe) or from repetitive microtrauma in athletes, particularly runners and hikers. In runners, the mechanism is the toe repeatedly striking the front of the shoe during downhill running or prolonged distance. The hallmark features: there is typically a history of injury or heavy athletic activity, the discoloration appears within hours to days of the causative event, the color is dark red to purple-black, and the discoloration grows out with the nail over 6–9 months (the nail growth duration).

A subungual hematoma causing significant pressure under the nail produces throbbing pain—sometimes severe enough to require drainage. Trephination (drilling a small hole through the nail to decompress the blood) provides immediate pain relief and can be performed by a podiatrist. For large hematomas (involving more than 25–50% of the nail area), evaluation for an underlying nail bed laceration or distal phalanx fracture may be appropriate. Small to moderate hematomas without significant pain do not require treatment—they resolve as the nail grows out.

Fungal Nail with Dark Discoloration

Onychomycosis (nail fungus) most commonly causes white, yellow, or brown discoloration with thickening and debris. However, certain fungal species—particularly Trichophyton violaceum and some Candida species—can produce dark brown to black discoloration. Fungal dark nails are distinguished by: thickening of the nail plate, crumbling or debris under the nail, multiple nails often affected, associated athlete’s foot, and the absence of a history of trauma. KOH preparation (examining nail scrapings under microscope) or nail culture confirms the diagnosis. Treatment is with oral antifungals (terbinafine or itraconazole) for the full course—topical antifungals alone do not effectively penetrate the nail plate for established infections.

Melanonychia and Subungual Melanoma

A dark streak running lengthwise under the nail (longitudinal melanonychia) requires careful evaluation. While benign streaks from nail nevi (moles) are common—particularly in individuals with darker skin tones—subungual melanoma presents identically and must be distinguished. Features concerning for melanoma include: Hutchinson’s sign (pigmentation extending from the nail onto the surrounding skin), width greater than 3mm, irregular color variation within the streak, recent change in a previously stable streak, and new-onset in an adult over 50. Any dark streak without a clear traumatic history that does not grow out with the nail should be evaluated by a podiatrist or dermatologist—nail matrix biopsy is the definitive diagnostic test.

Preventing Runner’s Black Toenail

Runners can significantly reduce subungual hematomas by: wearing properly fitted running shoes with adequate toe box space (thumb’s width between the longest toe and the shoe end), replacing shoes before they are excessively worn down, lacing shoes to prevent the foot from sliding forward (especially downhill), trimming toenails straight across and not too short, and wearing moisture-wicking socks. Toenails that are too long repetitively strike the shoe; those cut too short leave the nail bed vulnerable to shoe pressure. If black toenails recur with every marathon, this usually indicates a fitting problem with the shoe rather than a training error.

Frequently Asked Questions

Visit Balance Foot & Ankle — Same-Day Appointments Available

Our podiatry team serves patients throughout Michigan including Howell, Brighton, and Bloomfield Township. If you’re dealing with heel pain, ingrown toenails, or a foot injury, we have same-day appointment availability.

Medical References & Sources

• American Podiatric Medical Association — Nail Disorders
• PubMed Research — Subungual Hematoma in Runners
• PubMed Research — Melanonychia and Nail Melanoma

Dr. Tom Biernacki, DPM is a board-certified podiatric surgeon at Balance Foot & Ankle in Howell and Bloomfield Township, Michigan. He evaluates and treats nail conditions including subungual hematoma, nail fungus, and suspicious nail pigmentation, and performs nail procedures including trephination and nail biopsy.