Brown toenails are usually caused by one of a few common problems: a fungal infection, dried blood from an injury, repeated friction from shoes, or pigment changes in the nail itself. Less often, brown discoloration can signal a medication side effect, an underlying health condition, or rarely, a type of skin cancer that starts under the nail. The cause matters because some of these are harmless and resolve on their own, while others need treatment.
Fungal Infections
Fungal nail infections, known clinically as onychomycosis, are one of the most frequent reasons toenails turn brown, yellow, or white. The most common culprits are a group of fungi called dermatophytes, but certain molds can produce distinctly brown or black pigment. Fungi like Scytalidium, Alternaria, and Exophiala are specifically associated with brownish or blackish nail discoloration, a pattern sometimes called fungal melanonychia.
A fungal infection typically starts at the tip or side of the nail and works its way toward the base. Along with the color change, you’ll often notice the nail becoming thicker, crumbly, or slightly lifted from the nail bed. The discoloration tends to be uneven and may involve only part of the nail at first. Fungal infections don’t resolve on their own. Over-the-counter antifungal creams rarely penetrate the nail plate well enough to clear the infection, so most people need a prescription oral antifungal or medicated nail lacquer, and treatment can take several months because toenails grow slowly.
Injury and Trapped Blood
Stubbing your toe, dropping something heavy on it, or even running in shoes that are slightly too small can cause bleeding underneath the nail. This trapped blood, called a subungual hematoma, initially looks red or purple but gradually darkens to brown or black as the blood dries and breaks down.
The brown spot stays put until the nail grows out completely, which takes six to nine months for a toenail. During that time, you’ll see the discolored area slowly migrate toward the tip of the nail. If the injury was severe enough, the nail may loosen or eventually fall off before a new one grows in. Most subungual hematomas don’t need treatment beyond basic care, but if blood pressure builds up under the nail and the pain is intense, a doctor can relieve it by creating a small hole in the nail to drain the fluid.
Friction From Footwear
You don’t need a single dramatic injury to develop brown toenails. Repeated low-level pressure from tight or narrow shoes can stimulate the pigment-producing cells in the nail matrix to ramp up melanin production. This condition, called frictional longitudinal melanonychia, shows up as a brown streak running lengthwise down the nail. It tends to appear over the area where the bony tip of the toe presses against the narrowing front of the shoe.
Runners and people who spend long hours on their feet in snug footwear are especially prone. Histological studies of affected nails show increased melanin deposits in both the nail plate and the underlying tissue. The discoloration is benign but can persist as long as the friction continues. Switching to shoes with a roomier toe box often prevents it from worsening.
Medication Side Effects
Certain medications can trigger brown pigmentation in the nails, typically appearing as horizontal or longitudinal bands. Chemotherapy drugs are the most well-known cause, including doxorubicin, methotrexate, cyclophosphamide, and hydroxyurea. Antimalarial medications like chloroquine and amodiaquine can also do it. With antimalarials, the brown color sometimes comes from iron deposits rather than melanin. The HIV medication zidovudine and the autoimmune drug infliximab have been linked to nail darkening as well.
Medication-related nail changes are generally harmless and often reversible once the drug is stopped, though it can take many months for the discolored nail to grow out. If you notice new nail discoloration after starting a medication, mention it to your prescriber so they can confirm the connection and rule out other causes.
Chronic Kidney Disease
A distinctive pattern called Lindsay’s nails can appear in people with chronic kidney disease. The bottom half of the nail closest to the cuticle turns white, while the outer portion takes on a reddish-brown color that doesn’t fade when you press on it. The brown band typically occupies 20% to 60% of the nail bed. Up to 40% of people with chronic kidney disease develop this pattern, likely driven by elevated levels of a hormone that stimulates melanin-producing cells.
Lindsay’s nails are not dangerous on their own, but they can be an early visible clue of kidney problems that haven’t been diagnosed yet. If you notice this half-and-half pattern on multiple nails, it’s worth getting your kidney function checked.
When a Brown Streak Could Be Melanoma
Subungual melanoma is a rare but serious cause of brown nail discoloration. It accounts for roughly 0.7% to 3.5% of all melanoma cases worldwide, but that percentage rises significantly in people with darker skin tones, where it can represent up to 30% of melanoma diagnoses. It most often appears as a brown or black longitudinal band on a single nail.
Not every brown streak is cause for alarm. Benign moles in the nail matrix also produce brown bands and are far more common, especially in people with darker skin. But certain features raise concern. Dermatologists use a set of visual criteria to distinguish the two:
- Adult onset: A new brown band appearing in adulthood, particularly after age 50
- Brown background: The entire nail plate appears pigmented rather than having a crisp, well-defined band against normal-colored nail
- Color on surrounding skin: Pigment spreading onto the skin around the nail (called Hutchinson’s sign)
- Single digit: Discoloration limited to one nail rather than appearing on several
Other warning signs include a band that is widening over time, irregular or blurred borders, nail splitting or crumbling, and bleeding. A pyramid-shaped band that widens toward the cuticle is also suggestive. Benign nail moles, by contrast, tend to have sharp, well-defined edges with normal-colored nail on either side. When there’s any doubt, a nail matrix biopsy is the gold standard for diagnosis. Dermatologists generally recommend that any unexplained single-digit brown streak in lighter-skinned individuals be biopsied. In darker-skinned individuals, where benign melanonychia is more common and often appears on multiple nails, the threshold for biopsy is guided by whether the streak shows any of the concerning features above.
Preventing Brown Toenails
Many causes of brown toenails are avoidable with basic foot care. Wearing properly fitted shoes is the single most important step. Feet change size over time, so measuring them periodically helps ensure your shoes aren’t too tight, especially in the toe box. If your shoes get wet, let them dry completely and consider using a disinfecting spray to limit fungal growth.
Keep your feet clean and dry, particularly between the toes where moisture accumulates. If you wear toenail polish regularly, remove it periodically so you can actually see the nail underneath and catch any color changes early. Wearing shoes around the house protects against the kind of minor toe injuries that lead to trapped blood. Inspecting your toenails every few weeks takes only a moment but makes it easier to notice changes when they’re still new, which matters most for the rare cases where a brown streak turns out to be something that needs prompt attention.