Does Black Toenail Go Away? Causes & When to Worry

Most black toenails are caused by blood pooling under the nail after an injury, and they grow out on their own over several months without treatment. Because toenails grow at roughly 1.6 mm per month, a fully darkened big toenail can take nine months to a year to look normal again. That said, not every dark toenail is a simple bruise. Fungal infections, certain medications, and in rare cases melanoma can all turn a nail black, and those causes do not resolve by simply waiting.

The Most Common Cause Is a Bruise Under the Nail

A subungual hematoma, which is the medical term for blood trapped between the nail plate and the nail bed, is by far the most frequent reason a toenail turns dark. It happens when something crushes or jams the toe: dropping a heavy object on your foot, stubbing your toe hard, or the repetitive micro-trauma that long-distance runners experience with every stride. The blood collects in the tight space beneath the nail, producing a deep purple, reddish-black, or blue-black patch that can cover part or all of the nail.

Because the nail plate is hard and translucent, the trapped blood stays visible for as long as it takes for that section of nail to grow from the base to the free edge. Toenails grow slowly compared to fingernails. A study of healthy young adults found that toenails averaged about 1.62 mm of growth per month, with the big toenail growing slightly faster than the smaller ones.

Since a big toenail is typically 15 to 20 mm long from cuticle to tip, simple math tells you a bruise near the base of the nail can take close to a year to fully disappear. Along the way, the dark spot gradually migrates toward the tip and eventually gets trimmed off. A bruise near the tip of the nail will leave faster, sometimes in just a few months. Either way, the nail underneath usually regenerates normally as long as the nail matrix, the growth center tucked under the cuticle, was not permanently damaged.

When Runners and Athletes Get Black Toenails

Runners have their own version of this problem, sometimes called “runner’s toe” or “tennis toe.” Unlike a single crushing injury, the damage comes from the toe repeatedly sliding forward and hitting the inside of the shoe with each foot strike. Over the course of a long run, thousands of these micro-impacts add up to enough bleeding under the nail to discolor it. Research on foot mechanics during running found that the risk of toenail bruising increases as the foot swells and warms, which happens progressively during exercise. By around the 10-kilometer mark, the combination of swelling and heat makes the toes more likely to jam against the front of the shoe.

A few practical steps can lower the risk. Choosing shoes with a taller and longer toe box gives the toes more room to move without striking the shoe’s upper. Keeping laces snug enough to prevent the foot from sliding forward, without cutting off circulation, also helps. Athletic socks that wick moisture and cool the foot reduce swelling. Some experienced runners buy shoes a half-size larger than their everyday pair specifically to account for the foot’s expansion during a long run.

For athletes who get recurrent black toenails, the discoloration itself is usually harmless and resolves with normal nail growth. The bigger concern is if the nail loosens and falls off entirely, which happens when the hematoma is large enough to separate the nail plate from the bed. Even in that case, a new nail usually grows in over six to twelve months, though the replacement nail sometimes comes in slightly thicker or ridged.

Draining the Blood Can Relieve Pain Fast

A fresh subungual hematoma often throbs painfully because of the pressure building under the nail. If you injure your toe and the pain is significant, a doctor can drain the blood through a procedure called nail trephination. This involves making a small hole in the nail plate, either with a heated needle, a small drill, or a cautery device, to let the trapped blood escape. The relief is usually rapid.

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Trephination works best within the first day or two, while the blood is still liquid. Once it clots and dries, draining it becomes difficult and the procedure is less useful. At that point, the main option is just waiting for the nail to grow out. If the hematoma covers more than about half the nail, or if there is reason to suspect a broken bone underneath, the doctor may want to remove the nail entirely to inspect and repair the nail bed. This is more common in severe crush injuries where the nail bed itself is torn.

Fungal Infections That Mimic a Bruise

Not all dark toenails are caused by trauma. Certain fungi can produce pigment that turns a nail brown, yellowish-black, or outright black, creating what is called fungal melanonychia. The most familiar fungal nail infections turn nails yellow, white, or chalky, so a dark-colored fungal infection can catch people off guard. Case reports have identified Candida, Aspergillus, Chaetomium, and Neoscytalidium species as fungi capable of producing dark nail pigmentation.

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Neoscytalidium dimidiatum, in particular, is recognized as a growing cause of dark-pigmented nail infections in tropical and subtropical climates.

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The good news is that fungal melanonychia typically clears up with antifungal treatment. In one case series, nail pigmentation improved in all patients treated with antifungal medications, though one patient developed a new darkened nail on a different finger after stopping treatment.

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A key difference between a traumatic bruise and a fungal infection: a bruise grows out distally in a fixed pattern without changing shape, while a fungal infection tends to persist or spread over time. If a black nail is not migrating toward the tip over weeks, or if the nail is also thickened, crumbly, or inflamed around the edges, a fungal cause is worth investigating. A doctor can take a nail clipping or scraping and send it for culture to identify the organism and guide treatment.

Medications and Systemic Causes

Certain drugs can trigger nail darkening as a side effect. Chemotherapy agents are among the best-known culprits, but antimalarials, some antibiotics like minocycline, and phenothiazine medications used in psychiatry have all been documented to cause nail pigmentation changes.

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The mechanism varies by drug. Some increase melanin production in the nail matrix. Others deposit pigment directly into the nail. Heavy metals like silver and arsenic can also darken nails through a combination of metal deposition and stimulated melanin production.

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Drug-induced nail discoloration usually affects multiple nails rather than just one, which is a helpful clue. It also tends to develop gradually rather than appearing overnight. In most cases, the discoloration fades after the medication is stopped, though it takes several months of new nail growth for the change to be visible. A clinical review of melanonychia notes that drug-induced pigmentation is one of several systemic or iatrogenic causes of increased melanin in the nail.

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People with darker skin tones are also more likely to develop benign longitudinal melanonychia, dark streaks running lengthwise through the nail, as a normal variation. These streaks are caused by the activation of melanocytes in the nail matrix without any underlying disease and are not a sign that anything is wrong.

When a Black Toenail Could Be Melanoma

This is the scenario that understandably worries people the most. Subungual melanoma is a type of skin cancer that grows under the nail and often first appears as a dark band or patch. It accounts for a small fraction of all melanomas, roughly 0.7% to 3.5%, but it is disproportionately dangerous because it frequently gets diagnosed late.

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A subungual melanoma will not go away on its own, and it does not grow out with the nail the way a bruise does. Instead, it persists, slowly widens, and may change in color or shape over weeks to months. There are several features that should prompt a visit to a dermatologist:

Hutchinson sign: Pigmentation that extends from the nail onto the surrounding skin fold. This has traditionally been considered a hallmark of subungual melanoma, though it can occasionally appear in benign conditions as well.
Longitudinal dark band: A streak of brown or black running from the base of the nail to the tip, especially one that is irregular in width, has blurred edges, or darkens over time.
Single nail involvement: A new dark streak appearing on one nail in an adult, without a history of trauma.
Nail destruction: The nail plate splitting, thinning, or breaking apart without an obvious injury or infection.
Ulceration: An open sore or bleeding from under the nail that does not heal.

In a dermoscopy study comparing subungual hematomas to nail-unit melanomas, features like Hutchinson sign, irregular longitudinal bands, and ulceration were found in the melanoma cases but not in any of the hematoma cases.

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It is worth knowing that subungual melanoma is more common in people of African, Asian, and Hispanic descent compared to lighter-skinned populations. Because it occurs in a location not typically included in routine skin checks, it often gets mistaken for a bruise or fungal infection for months before anyone investigates further. If you have a dark toenail that appeared without clear trauma and has not changed position over a few months, getting it checked is worthwhile.

How Doctors Tell These Causes Apart

Distinguishing a bruise from something more serious can sometimes be done on appearance alone, but not always. Dermoscopy, which uses a handheld magnifying device with polarized light, is one of the first tools dermatologists reach for. A subungual hematoma under dermoscopy tends to show reddish-purple to purplish-black coloring and is often located in the distal two-thirds of the nail. A study comparing hematomas and melanocytic lesions found that hematomas occupied the distal two-thirds of the nail in about half of cases, while melanocytic lesions involved the entire nail over 90% of the time. Hematomas also showed a wider variety of colors including purple, red, and blue-black, while all melanocytic lesions appeared brown.

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One practical home test that doctors sometimes mention is the “grow-out” observation. Take a photo of the dark nail with a ruler next to it. Check again in a month or two. If the dark area has moved distally with normal nail growth, it is almost certainly a hematoma. If the dark area stays anchored at the base or has expanded, further evaluation is needed.

For fungal melanonychia specifically, dermoscopic features like white or yellow streaks, reverse triangular patterns, and scaling on the nail surface help distinguish it from melanoma. A comparison study found all four of these fungal-associated patterns to be positive predictors of fungal origin, while melanoma-associated signs like Hutchinson sign, irregular triangular shapes, and ulceration pointed away from a fungal cause.

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When dermoscopy is inconclusive and melanoma cannot be ruled out, the gold standard is a nail biopsy. This involves sampling the nail matrix, the tissue that produces the nail plate, and examining it under a microscope. Nail biopsies are not done casually, since they can cause permanent nail changes like ridging or splitting, but they are the definitive way to confirm or exclude melanoma.

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Dark Streaks That Are Completely Normal

Longitudinal melanonychia, a dark streak running the length of the nail, is not always a sign of disease. In fact, it can be a perfectly normal finding. A clinical review by a panel of nail experts noted that benign melanonychia can result from either external causes (blood, bacteria, fungus) or internal ones, including simple overproduction of melanin by a normal number of melanocytes in the nail matrix. This overproduction can be triggered by normal physiological variation, local inflammation, or systemic factors.

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In people with darker skin, benign longitudinal melanonychia is common enough that it can be considered a normal variant. Multiple nails may be affected, the bands tend to be uniform in color and width, and they remain stable over time. This is distinctly different from the irregular, widening, single-nail presentation that raises concern for melanoma. A dermatoscopy review notes that non-continuous nail pigment changes, those affecting only part of the nail rather than running its full length, are most often caused by something other than melanocyte growth, such as old hemorrhage or fungal infection.

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A Simple Decision Framework

Given how many things can darken a toenail, it helps to have a rough mental checklist for deciding whether to wait it out or seek evaluation:

Clear trauma history: You remember dropping something on your toe or your shoes were too tight during a long run. The dark spot appeared within hours to days. Pain came first, then discoloration. This is almost certainly a subungual hematoma. Wait and watch it grow out.
Gradual onset, thickened nail: The nail darkened slowly, and it is also crumbly, thick, or separated from the bed. The surrounding skin may be red or scaly. Think fungal infection. See a doctor for a nail culture and treatment.
Multiple nails affected: Dark bands or discoloration on several toenails at once, especially if you recently started a new medication. Drug-induced pigmentation or benign ethnic melanonychia are likely. Mention it to your prescribing doctor.
Single dark streak, no trauma: A new brown or black band on one nail that appeared without injury and does not migrate. Especially concerning if the band is irregular in width, the pigment extends onto the skin around the nail, or you notice any bleeding or ulceration. See a dermatologist promptly.

Most black toenails fall into the first category and resolve without intervention. The discoloration is cosmetically annoying and the wait is long, but the outcome is a normal-looking nail. For the minority of cases that fall into other categories, the key is noticing when the nail is not behaving the way a simple bruise should. A bruise stays the same size and moves toward the tip. Anything that grows, spreads, stays put, or comes with changes to the nail’s texture deserves professional attention.

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