A brown spot on your toenail is almost always caused by something harmless: a bruise, repeated friction from shoes, a small mole in the nail bed, or a fungal infection. These benign causes account for the vast majority of cases. Rarely, a brown spot can signal melanoma under the nail, so knowing what to watch for matters.
The Most Common Cause: A Bruise Under the Nail
Stubbing your toe, dropping something on your foot, or wearing tight shoes during a long run can trap blood beneath the nail. This pooled blood, called a subungual hematoma, typically looks like a reddish-brown or dark purple-black spot. You may not even remember the injury that caused it, especially if it was mild or repetitive (think: shoes that are just slightly too small).
A bruise under the nail has a few telltale features. The spot usually sits closer to the tip of the nail rather than the base, and its nearest edge tends to be rounded and well-defined. Over time, the spot moves forward as your nail grows out. Toenails grow about 1.5 millimeters per month, so it can take 12 to 18 months for a spot near the base to fully disappear. If you want to track it, you can scratch a thin line across the nail just behind the spot and check over a few weeks to confirm it’s migrating forward.
Repeated Trauma and Friction
You don’t need a single dramatic injury to get a brown toenail. Repeated low-grade trauma, like the friction of your longest toe hitting the front of your shoe during walking or running, can stimulate the pigment-producing cells in your nail bed. This is especially common in runners and hikers and often affects the big toe. The discoloration may appear as a diffuse brownish patch rather than a distinct spot, and it tends to recur if the source of friction isn’t addressed.
Fungal Infection
Most fungal nail infections turn nails yellow or white, but certain less common fungi can produce brown or black pigmentation. These species are rarer, but they do exist. The key difference between a fungal infection and other causes is what happens to the nail itself: fungal infections typically make the nail thick, crumbly, or brittle. The nail may crack, separate from the nail bed, or accumulate debris underneath. If your brown spot comes with any of these texture changes, a fungal infection is a strong possibility. A doctor can confirm this with a simple nail clipping sent to a lab.
Natural Pigmentation
People with medium to dark skin tones are more likely to develop brown or black bands in their nails simply due to higher baseline melanin production. This type of pigmentation often appears as a vertical stripe running the length of the nail and can show up on multiple nails at once. It’s completely benign and requires no treatment. Age spots (lentigos) can also form in the nail bed, producing small brown marks similar to the ones that appear on sun-exposed skin.
Small moles can develop in the nail matrix, the tissue at the base of the nail where new nail cells form. These moles produce a regular, even brown band that stays consistent in color and width over time. They’re harmless but worth keeping an eye on for any changes.
Medications and Other Triggers
Certain medications can cause brown or black lines or bands across your nails. Chemotherapy drugs, antimalarials like hydroxychloroquine, and some HIV medications are the most common culprits. The discoloration typically appears on multiple nails and develops after starting the medication. Pregnancy, vitamin B12 deficiency, and thyroid conditions can also trigger nail darkening. In these cases, the discoloration often improves once the underlying cause is addressed.
When a Brown Spot Could Be Melanoma
Melanoma under the nail is rare. It accounts for about 3% of melanomas in light-skinned people, though it represents a higher proportion (around 30%) of melanomas diagnosed in people with dark skin. Despite its rarity, it’s the reason any new or changing brown mark on a toenail deserves attention.
Dermatologists use a set of criteria to screen for nail melanoma, and you can apply the same logic at home:
- Age and ancestry: Peak incidence is between ages 40 and 70. It’s more common in African American, Asian, and Native American populations.
- Band characteristics: A brown-to-black stripe wider than 3 millimeters, with uneven color or irregular borders, is more concerning than a thin, uniform line.
- Change over time: A stripe that widens, darkens, or becomes uneven warrants evaluation. A benign mark stays stable.
- Digit involved: The big toe and thumb are the most commonly affected.
- Pigment spreading beyond the nail: If brown or black color extends onto the skin around the nail (the cuticle or the skin alongside the nail), this is called Hutchinson’s sign and is a hallmark of melanoma.
- Family history: A personal or family history of melanoma raises the level of concern.
The shape of the pigmentation also matters. Melanoma often appears as a lengthwise stripe that becomes wider toward the base of the nail, creating a funnel shape. Benign moles produce regular, evenly spaced parallel lines. A bruise, by contrast, sits in a defined blob that moves forward with growth.
What Happens During an Evaluation
If you bring a brown toenail spot to a dermatologist, the first step is usually dermoscopy, a painless exam using a magnifying device with polarized light. Under dermoscopy, a bruise shows distinct reddish-to-black blotches with a rounded proximal edge. A benign mole shows regular parallel brown lines on an even background. Melanoma shows irregular lines that vary in color, spacing, and thickness, often with disruption of the normal parallel pattern.
If the diagnosis is unclear after dermoscopy, the doctor may try treating a suspected cause first. For example, if a fungal infection seems likely, a course of antifungal treatment may be started. If the spot doesn’t respond to treatment or continues to change, a biopsy becomes the next step. This involves removing a small piece of tissue from the nail bed to examine under a microscope. When possible, doctors avoid biopsying the part of the nail matrix responsible for forming the visible nail surface, since scarring there can cause a permanent ridge or groove in the nail.
Tracking a Spot at Home
Most brown spots on toenails are harmless, but monitoring is simple and worth doing. Take a clear, well-lit photo of the nail every month using the same angle and distance. Look for three things: whether the spot is moving forward with nail growth (a good sign), whether it’s staying the same size and color (also reassuring), or whether it’s widening, darkening, or spreading to the surrounding skin (a reason to get it checked). Given that toenails take 12 to 18 months to fully grow out, patience is part of the process. A bruise or minor trauma mark should be noticeably closer to the tip after a few months.