A brown spot on your nail is almost always caused by one of a few common things: a bruise under the nail, a fungal infection, or a natural increase in pigment from the cells at the base of your nail. Less often, it can signal a nutritional deficiency, a medication side effect, or, rarely, a type of skin cancer that starts under the nail. The cause usually becomes clear once you consider what the spot looks like, whether it’s changing, and what else is going on with your health.
Bruising Under the Nail
The single most common cause of brown or black discoloration on a nail is a subungual hematoma, which is a small pocket of blood trapped between the nail and the skin underneath. You might remember stubbing your toe, slamming a finger in a door, or wearing tight shoes, but sometimes the injury is so minor you don’t recall it happening at all. The trapped blood starts out dark red or purple and gradually turns brown or black as it ages.
The key feature of a bruise is that it moves. Because the blood gets locked into the nail plate, it slowly travels toward the tip of your finger or toe as the nail grows out. Fingernails grow roughly 3 to 4 millimeters per month, so a bruise near the base can take several months to fully disappear. This slow pace often alarms people, but if the spot is clearly migrating outward over weeks, that’s a strong sign it’s just old blood working its way out.
Fungal Infections
Fungal nail infections don’t always look white or yellow. Certain fungi produce dark pigments that turn the nail brown or even black. The most common culprit is a fungus called Trichophyton rubrum, responsible for about 55% of fungal melanonychia cases. Other species, particularly dark-pigmented molds, can also cause brown discoloration.
A fungal infection usually comes with other changes beyond just color. You might notice the nail thickening, becoming brittle or crumbly, lifting away from the nail bed, or developing white spots and surface scaling alongside the brown areas. About 31% of fungal melanonychia cases show multicolor pigmentation rather than a single uniform brown. If the brown spot is accompanied by any of these textural changes, a fungal infection is a strong possibility, and it’s treatable with antifungal medication.
Normal Pigmentation and Skin Tone
In people with darker skin tones, brown lines or bands running lengthwise down the nail are extremely common and completely harmless. This is sometimes called racial or ethnic melanonychia. It happens because the pigment-producing cells at the base of the nail are naturally more active. The prevalence varies dramatically by ethnicity: brown longitudinal bands appear in 77% to 100% of African Americans, 10% to 20% of Japanese and Asian individuals, and only about 1% of white people.
These bands typically appear on multiple nails rather than just one, and they tend to stay stable over time. If you’ve had faint brown lines on several nails for years and they haven’t changed much, this is the most likely explanation.
Vitamin Deficiencies
Low vitamin B12 is a well-documented cause of nail darkening. When B12 levels drop, a chain reaction in the pigment cells ramps up melanin production. Lab studies show that B12 deficiency activates the enzyme responsible for making melanin while simultaneously reducing the antioxidant that normally keeps that enzyme in check. The result is excess pigment deposited into the nail plate, usually appearing as longitudinal brown or bluish-brown bands.
The good news is that B12-related nail changes are reversible with supplementation. Vitamin D deficiency and severe protein malnutrition can also trigger nail pigmentation, though these are less common in well-nourished populations. Endocrine conditions like Addison’s disease, Cushing’s syndrome, and hyperthyroidism occasionally cause darkening across multiple nails as well.
Medications That Darken Nails
Certain medications can cause brown or blue-brown nail discoloration as a side effect. Chemotherapy drugs are the most frequent offenders, but the list also includes some HIV medications, certain antibiotics (minocycline in particular can cause a muddy brown color), and antimalarials. Drug-induced nail changes usually affect several nails at once and often come alongside darkening of the skin or mucous membranes. If you started a new medication in the months before the spot appeared, that timing is worth noting.
When a Brown Spot Could Be Melanoma
Subungual melanoma, a type of skin cancer that starts in the nail matrix, is rare but serious. It accounts for a small fraction of all melanomas, yet it’s frequently diagnosed late because people assume the dark mark is a bruise or harmless streak. About 65% of nail melanomas first appear as a dark vertical band on a single nail.
Dermatologists use a set of criteria sometimes called the ABCDEF guidelines to flag suspicious nail pigment:
- Age and ancestry: most common between ages 50 and 70, and in people of African, Asian, Native American, or Hispanic descent
- Band characteristics: a brown or black band wider than 3 millimeters with blurry or irregular borders
- Change: the band is getting wider, darker, or changing shape over weeks to months
- Digit: the thumb, big toe, and index finger are the highest-risk locations
- Extension: pigment spreading from the nail onto the surrounding skin (called the Hutchinson sign)
- Family history: a personal or family history of melanoma
The Hutchinson sign, where brown or black color bleeds onto the skin around the nail fold, has long been considered a hallmark of nail melanoma. That said, about 30% of people with benign conditions like nail moles or ethnic melanonychia can show similar-looking pigment around the nail, so it’s not an automatic diagnosis. What matters most is whether the pigment is new, solitary, and changing.
How to Tell These Apart
A few simple observations can help you sort through the possibilities. A bruise will migrate toward the nail tip over weeks and eventually grow out entirely. A fungal infection usually changes the nail’s texture, not just its color. Ethnic melanonychia tends to show up on multiple nails and stays stable for years. Medication-related darkening lines up with the start of a new drug.
The features that should prompt a dermatologist visit are a single dark band on one nail that appeared without obvious injury, any band that is widening or darkening over time, pigment that extends onto the skin around the nail, and any nail discoloration accompanied by the nail cracking, splitting, or distorting. A dermatologist can examine the nail under magnification and, if needed, take a small biopsy from the nail matrix to rule out melanoma. Early-stage nail melanoma is highly treatable when caught before it spreads, which makes paying attention to changes the most important thing you can do.