Black Toenail Causes & Care 2026 | Podiatrist

Quick answer:A black toenail is most often a bruise — blood trapped under the nail after injury or tight shoes — that grows out over months. Less commonly it is fungus or, rarely, melanoma, so any dark nail that appears without an injury or keeps changing should be checked by a podiatrist. Call (810) 206-1402.

A black toenail is alarming the first time you see it, but most cases are completely benign. As a podiatrist, I evaluate black toenails every week — the vast majority are from blood under the nail caused by injury or shoe friction. However, a small minority have a different cause that needs attention. Here’s how to tell the difference.

1. Subungual Hematoma (Blood Under the Nail) — Most Common

A subungual hematoma is pooled blood between the nail plate and nail bed. It appears as a dark red, maroon, or black discoloration under the nail. It’s caused by direct trauma (dropping something on the toe, stubbing), or repetitive microtrauma from running in shoes that are too short or too narrow.

Runner’s toenail is the most common athletic cause — the second toe (which is often longer than the big toe) repeatedly strikes the shoe’s toe box during downhill running, creating a chronic hematoma. Ultramarathoners and hikers frequently develop black toenails on multiple toes.

Presentation: black or dark red, usually painful immediately after injury, then becomes painless as blood dries. The nail may eventually loosen and fall off as new nail grows underneath. No treatment is needed for most cases. For large painful hematomas, a podiatrist can drain the blood by creating a small hole in the nail plate (trephination) — this provides immediate pain relief.

2. Black Toenail Fungus

Toenail fungus (onychomycosis) typically causes yellow, brown, or white discoloration, but certain fungal species produce dark pigments. Aspergillus niger and some dermatophytes can cause black or very dark brown nail changes. The key distinguishing features of fungal black nail: gradual onset over months, nail thickening and brittleness, crumbling at the free edge, and usually no history of trauma.

3. Melanonychia (Dark Pigment Streaks)

Longitudinal melanonychia is a brown or black stripe running from the cuticle to the free edge of the nail. It occurs when melanocytes (pigment-producing cells) in the nail matrix become active. In darker-skinned individuals, melanonychia is very common and almost always benign. In lighter-skinned individuals, a new melanonychia stripe warrants evaluation.

Benign causes of melanonychia include medications (hydroxychloroquine, chemotherapy), systemic conditions (Addison’s disease), and nail matrix nevi (moles under the nail).

4. Subungual Melanoma — Rare But Important

Subungual melanoma (melanoma under or in the nail) is rare — about 0.7-3.5% of all melanomas — but it is the most serious cause of black toenail. It presents as a dark longitudinal stripe (melanonychia) that is typically wider than 3mm, irregular in color, and may spread to involve the cuticle skin (Hutchinson’s sign). The big toenail and thumb nail are most commonly affected.

Key red flags that distinguish melanoma from benign melanonychia: the stripe is widening over time, color is uneven (multiple shades of black, brown, gray), the pigment extends onto the skin around the nail, or you notice any nodule or ulceration. If you have any of these, see a podiatrist or dermatologist promptly for a nail biopsy.

How Black Toenails Are Treated

Treatment depends entirely on the cause. For traumatic hematomas, most resolve on their own; painful ones can be drained in office in under 5 minutes. For fungal black nails, treatment is the same as yellow fungal nails — oral terbinafine or our laser fungal treatment is the most effective option. For suspected melanonychia or melanoma, a nail matrix biopsy is performed to determine the diagnosis — this is a minor surgical procedure done under local anesthetic.

In-Office Treatment at Balance Foot & Ankle

We evaluate and treat all nail conditions including black toenail, nail fungus, and nail lesions. If you have a black toenail without a clear injury history or one that has been present for months, don’t wait — early evaluation is important. We offer nail biopsies, laser fungal treatment, and hematoma drainage in-office.

Same-day appointments available. (810) 206-1402 | Book online

Frequently Asked Questions

Will a black toenail fall off?

If the black toenail is from a subungual hematoma, the nail often loosens and falls off over 2-4 months as new nail grows from the matrix and pushes the injured nail forward. The new nail growing underneath is completely normal. If the nail falls off without new nail growing to replace it, or if the nail bed looks abnormal, see a podiatrist.

How do I prevent black toenails from running?

The main prevention strategies are: buy running shoes a half-size to full size longer than your regular shoe (feet swell and slide forward during runs), keep toenails trimmed short straight across, wear moisture-wicking socks to reduce friction, and lace shoes firmly enough to prevent the foot from sliding forward on downhills. If you frequently get black toenails on the second toe, you may have Morton’s toe (a longer second toe) — a podiatrist can assess whether orthotics would help.

Is a black toenail always caused by injury?

No — while trauma is the most common cause, black toenails can also result from fungal infection (producing dark pigments), medication side effects, systemic diseases, benign nail matrix moles (nevi), and rarely melanoma. If you have no memory of injuring your toe and a black toenail appears, that warrants a podiatry visit rather than a “watch and wait” approach.