A brown spot on a toenail is a bruise in the overwhelming majority of cases, but fungal infections, medication side effects, normal ethnic pigmentation, and rarely melanoma can all produce similar discoloration. The challenge is that these causes can look strikingly alike to the naked eye, and the one you least want to miss, subungual melanoma, accounts for only about 2% of all melanoma cases yet carries a significantly worse prognosis when caught late.1PubMed. Epidemiology and prognosis of subungual melanoma Knowing which features point toward each cause can help you decide whether to wait, treat, or get to a dermatologist quickly.
Bruises Under the Nail
The single most common reason for a brown or dark spot under a toenail is a subungual hematoma, which is blood trapped between the nail plate and the nail bed. Stubbing a toe, dropping something on your foot, wearing tight shoes on a long hike, or repetitive impact during running can all burst tiny blood vessels beneath the nail. The resulting spot often starts reddish-purple and shifts toward brown or black as the blood breaks down over the following days. Unlike a bruise on your skin, a subungual hematoma has nowhere to spread, so the color stays concentrated in a well-defined patch or streak.
If you look closely at a fresh hematoma with a magnifying lens, the color tends to appear as a uniform, homogeneous blotch rather than as distinct lines or bands. Dermoscopic studies confirm that a homogeneous color pattern shows up in essentially all subungual hematomas, often accompanied by small round spots of color called globules.2PubMed Central. Dermoscopic diagnosis of subungual hematoma: new observations This is a useful distinction: bruises look like blots; melanoma tends to look like lines.
Most bruises under the nail grow out with the nail itself. Toenails grow slowly, roughly a millimeter or so per month, so it can take six to nine months for the spot to reach the free edge and disappear. During that time, the spot should migrate forward with the nail rather than staying in one place. A hematoma that does not grow out, or that stays fixed at the base of the nail while the nail grows past it, is behaving more like pigment being continuously produced rather than trapped blood, which is a red flag.
When Fungus Turns Nails Dark
Fungal nail infections, or onychomycosis, are extremely common and usually produce thickened, yellowish, or crumbly nails. Most people picture nail fungus as a white or yellow problem, not a brown one. But certain fungi, particularly a group called dematiaceous molds, produce melanin in their cell walls and can deposit brown-to-black pigment in the nail plate.3PubMed Central. Onychomycosis caused by dematiaceous fungi: A four-year study on agricultural workers of Assam, India These molds are less common culprits than the standard dermatophytes, but they are especially worth knowing about because the color they create can easily be confused with melanoma.
A few clues point toward fungus rather than something more serious. Fungal discoloration usually involves changes to the nail’s texture along with color: the nail may be thicker, brittle, lifted from the bed, or surrounded by debris. The discoloration often starts at the tip or sides and creeps toward the base, rather than appearing as a defined streak running the full length of the nail. And fungal infections frequently affect more than one nail, while melanoma almost always involves a single digit. That said, melanoma has been misdiagnosed as a fungal infection often enough that the combination of dark pigment and nail dystrophy should prompt a closer look rather than automatic treatment with antifungal cream.
Bacterial infections can also change nail color, though they tend toward green rather than brown. The bacterium Pseudomonas aeruginosa, for instance, produces a green pigment and is the classic cause of “green nail syndrome.”4The Avicenna Medical Journal. Green Nail Syndrome, A Rare Infection of Pseudomonas aeruginosa in The Nail Green discoloration is not the focus here, but it is worth mentioning because patients sometimes describe greenish-brown nails and wonder if the color is melanoma-related. If the dominant hue is green, infection is far more likely.
Recognizing Subungual Melanoma
Subungual melanoma is rare, but its rarity is exactly what makes it dangerous. Because neither patients nor primary care doctors see it often, it frequently gets dismissed as a bruise, fungus, or other benign condition. One case report estimates that roughly 85% of subungual melanomas are initially misdiagnosed, and the reported five-year survival when the cancer is caught late is only about 27%.5PubMed Central. A Misdiagnosed Case of Malignant Melanoma in an Infected Nail: A Case Report It arises from the melanocytes in the nail matrix, the tissue at the base of the nail that produces the nail plate.6PubMed. Subungual melanoma: Management in the modern era
Dermatologists have developed a screening mnemonic, sometimes called the ABCDEF rule, to help spot nail melanoma earlier. In this system, A refers to adult age, with peak incidence in the fifth through seventh decades; B stands for a brown-to-black band that is 3 mm or wider with irregular borders; C stands for a change in the band over time, or no change in a nail problem despite adequate treatment; D highlights that the thumb, index finger, and great toe are the digits most commonly affected; E stands for extension of pigment onto the surrounding skin fold; and F covers family or personal history of melanoma.7PubMed. The ABC rule for clinical detection of subungual melanoma A simplified version of this approach focuses on whether a single dark band on one digit in an adult shows brown bands on a brown background, or color spreading into the surrounding skin, and has demonstrated high sensitivity for catching early-stage melanoma.8PubMed Central. Early Detection of Subungual Melanoma In Situ: Proposal of ABCD Strategy in Clinical Practice Based on Case Series
The most visually alarming feature is pigment spreading beyond the nail itself onto the cuticle or the skin surrounding the nail. This is called the Hutchinson sign, first described in 1886, and it has long been considered strongly suggestive of subungual melanoma. Dermatologists still recommend biopsy whenever it appears. But the Hutchinson sign is not an automatic melanoma diagnosis. False positives occur with ethnic pigmentation, certain medications like minocycline, radiation therapy, trauma, systemic conditions like Cushing syndrome or hyperthyroidism, and benign congenital nevi.9Actas Dermo-Sifiliográficas. Is Hutchinson’s Sign Pathognomic of Subungual Melanoma? So the sign raises the alarm but does not close the case.
Under a dermatoscope, the tool dermatologists use to examine pigmented lesions at high magnification, melanoma tends to show brown-to-black longitudinal lines that are irregular in spacing, thickness, and color. In one study, this irregular line pattern showed up in 95% of melanoma cases and was significantly more common in melanoma than in every other diagnosis.10JAMA Dermatology. Dermoscopic Examination of Nail Pigmentation A subungual hematoma, by contrast, looks homogeneous and blobby rather than streaky. That visual difference is one of the most reliable ways to tell the two apart without a biopsy.
Medications and Medical Conditions That Darken Nails
Certain drugs can trigger brown or black streaks in the nails, a phenomenon called drug-induced melanonychia. Chemotherapy is the most well-known cause, with melanonychia appearing in a large proportion of patients undergoing treatment. One study of cancer patients receiving chemotherapy found nail changes in about 38% of patients, and among those with nail changes, melanonychia was the most common finding by a wide margin, occurring in roughly 84% of affected patients.11PubMed Central. Nail Changes Caused by Chemotherapy among Cancer Patients: A Cross-Sectional Study of Northwest Rajasthan Platinum-based drugs, taxanes, and 5-fluorouracil are among the agents most frequently implicated.12PubMed Central. Chemotherapy-related striate melanonychia: a case report The discoloration typically appears as longitudinal bands affecting multiple nails and fades once treatment ends, though the slow growth rate of nails means it can persist for months afterward.
Beyond chemotherapy, antimalarials like hydroxychloroquine, certain antiretroviral drugs, and antibiotics such as minocycline can produce similar nail darkening. The practical point for patients is that if you are taking any of these medications and notice new brown lines in your nails, mention it to your prescribing physician, but it is far more likely to be a drug side effect than melanoma. Involvement of multiple nails at once, in particular, points away from melanoma.
A handful of systemic diseases can also darken the nails. Addison’s disease, a condition in which the adrenal glands do not produce enough cortisol, can cause longitudinal melanonychia as one of its earliest visible signs, sometimes before more classic symptoms like fatigue and low blood pressure appear. Hyperthyroidism and Cushing syndrome have also been linked to nail pigmentation changes. These are uncommon causes, but they are worth considering if brown bands appear across several nails without an obvious explanation and do not match any of the more common culprits.
Brown Bands in Darker Skin Tones
Longitudinal melanonychia, the formal name for pigmented streaks running the length of the nail, is extremely common in people with darker skin and is almost always benign. The brown-to-black bands affect an estimated 15 to 20% of Asian individuals and as many as 70% of Black adults over the age of 20. The streaks result from normal melanocyte activity in the nail matrix and tend to appear on digits that experience more use or trauma. This kind of ethnic or racial pigmentation is often present on multiple nails and typically remains stable in width and color over time.
The clinical challenge is that subungual melanoma is disproportionately common among Black, Asian, and Hispanic populations relative to their overall melanoma rates. In these groups, acral melanoma, the subtype that includes nail melanoma, can account for up to a third of all melanoma cases, compared with a much smaller share in white populations.7PubMed. The ABC rule for clinical detection of subungual melanoma Because benign pigmented bands are also very common in the same populations, both patients and clinicians sometimes dismiss a changing band as normal variation. The key difference is stability: benign ethnic pigmentation tends to appear early in life, involve more than one nail, and stay the same width. A new or widening single-nail band in an adult, especially one with irregular coloring or pigment spilling onto the surrounding skin, should be evaluated regardless of the person’s racial background.
Benign Melanocytic Lesions
Not every pigmented streak that is more than a bruise and less than melanoma is a medical emergency. Two benign growths in the nail matrix can produce longitudinal melanonychia that looks worrisome but is harmless. Nail matrix nevi are moles that arise in the matrix tissue and are most common in children and young adults. Nail lentigo is a similar flat area of increased pigment, typically seen in adults. Both conditions produce a single pigmented band on one nail. The clinical appearances can be nearly identical, and the main differences are the patient’s age and what the tissue looks like under a microscope: nevi contain clusters of melanocyte cells, while lentigo does not.13Chinese Journal of Plastic and Reconstructive Surgery. Longitudinal Melanonychia: How to Distinguish a Malignant Condition from a Benign One
In children, longitudinal melanonychia is overwhelmingly benign. Dermoscopic features that would be suspicious for melanoma in adults, like gray band color or irregular borders, show up frequently in children’s benign nail lesions as well, making the standard adult screening criteria unreliable for younger patients.14PubMed Central. Dermoscopic Evaluation of Longitudinal Melanonychia in Children: A Prospective Study This is one reason pediatric dermatologists tend to monitor pigmented nail bands in children more conservatively, often watching for change over time rather than rushing to biopsy.
Why Misdiagnosis Matters
Delayed diagnosis is the central danger with subungual melanoma, and the numbers behind it are sobering. In a study of foot melanomas, about 30% of patients were initially given a wrong diagnosis. Common misdiagnoses included chronic wounds, nevi, hematoma, fungal infections, warts, and paronychia. That misdiagnosis delayed proper treatment by a median of nine months. The consequences were measurable: the five-year disease-free survival rate was roughly 48% in misdiagnosed patients compared with about 73% in those diagnosed correctly at first, and overall five-year survival was about 64% versus 88%.15PubMed Central. Initial misdiagnosis of melanoma located on the foot is associated with poorer prognosis
The lesson is not that every brown toenail needs an urgent biopsy. Most brown spots on toenails are harmless, and biopsying a nail matrix is a real procedure with a real recovery. The lesson is that certain features should prompt a dermatology referral rather than a watch-and-wait approach or a trial of antifungal treatment. A single-digit band that is widening, changing color, has irregular lines, or shows pigment on the surrounding skin clears that threshold. So does a dark spot that does not grow out with the nail over several months. If you are uncertain, a dermatologist can use a dermatoscope to examine the nail in minutes and decide whether a biopsy is warranted.
What a Nail Biopsy Involves
When a dermatologist suspects something beyond a bruise or fungus, a biopsy of the nail matrix is the only way to get a definitive answer. Several techniques exist. A punch biopsy uses a small cylindrical blade, typically 3 mm or smaller, to take a core of tissue from the matrix. An excisional biopsy removes a small ellipse of tissue. A shave or tangential biopsy takes a thinner slice and may be appropriate for superficial lesions.16PubMed Central. Nail Biopsy: A User’s Manual The nail plate often needs to be partially or fully removed first to access the matrix underneath.
Recovery depends on the technique used and how much tissue was taken. Small punch biopsies typically heal well with minimal long-term nail changes. Larger excisions from the matrix carry a higher risk of permanent nail dystrophy, meaning the nail may grow back with a ridge, split, or thin area. For this reason, dermatologists generally try to use the smallest biopsy that will provide an adequate tissue sample. The temporary discomfort and cosmetic trade-off of a biopsy is trivial compared with the risk of letting a melanoma grow undetected for months. If your dermatologist recommends a biopsy, the risk-benefit calculation overwhelmingly favors going through with it.
A Practical Checklist for Brown Toenail Spots
Pulling together the features discussed above, there are a handful of questions worth asking yourself before deciding whether to book a doctor visit or simply keep an eye on things:
- Did you injure the toe? If you can recall a specific impact and the spot appeared shortly after, a hematoma is likely. Watch for it to grow out with the nail over the next several months.
- Is the nail thickened, crumbly, or lifted? Texture changes alongside discoloration suggest fungal infection. Over-the-counter treatments exist, but prescription antifungals are often needed for toenails.
- Is the spot a streak or a blotch? A well-defined longitudinal band running from the cuticle to the nail tip behaves differently from a round blotch. Bands can be benign, but new single-nail bands in adults warrant monitoring at a minimum.
- Is pigment spreading onto the skin? Color extending beyond the nail plate onto the cuticle or nail fold is a reason to see a dermatologist promptly.
- Are multiple nails affected? Brown bands on several nails are far more likely to reflect ethnic pigmentation, medication effects, or a systemic condition than melanoma.
- Has the spot changed? Growth in width, darkening, or development of irregular color over weeks to months is the single most important warning sign, regardless of what the spot looks like at any one moment.
None of these questions replace a professional evaluation for a spot that worries you. But they can help you gauge urgency and describe the spot more accurately when you do see a doctor, which itself speeds up the diagnostic process.