Brown nails have several possible causes, ranging from harmless staining and minor trauma to fungal infections, nutritional deficiencies, and rarely, skin cancer. The pattern of discoloration matters a lot: whether it’s a uniform color change, a vertical streak, or a band across the nail points toward very different explanations.
External Staining
The simplest explanation is surface-level staining from chemicals you regularly touch. Nicotine and tar from cigarettes penetrate the skin’s pores over time, leaving yellowish-brown stains on the fingers used to hold a cigarette, pipe, or cigar. Hair dye, self-tanner, certain spices like turmeric, and nail polish (especially darker shades worn without a base coat) can all leave behind a brownish tint.
Staining tends to be uniform across the nail surface rather than forming a distinct line or band. It usually affects multiple nails on the same hand, and it fades as the nail grows out or can be scrubbed off with gentle cleaning solutions. If the brown color only appeared after you started using a new product or chemical, staining is the likely culprit.
Fungal Infections
Nail fungus often starts as a white or yellow-brown spot under the tip of a fingernail or toenail. As the infection deepens, the nail discolors further, thickens, and begins to crumble at the edge. The most common cause is a group of fungi called dermatophytes, though yeasts and molds can also be responsible.
Fungal nail infections are more common in toenails than fingernails because feet spend more time in warm, moist environments like shoes. You’ll typically notice the texture change alongside the color change: the nail feels brittle, rough, or unusually thick. It may also lift away from the nail bed or give off a slight odor. These infections don’t resolve on their own and generally need antifungal treatment that can take several months to fully clear.
Bruising Under the Nail
A subungual hematoma, or bruise beneath the nail, happens when you stub a toe, slam a finger in a door, or put repeated pressure on a nail (common in runners). Blood pools under the nail plate and can appear dark brown, reddish-purple, or black. It’s usually painful at first and clearly linked to an injury you remember.
The key feature of a bruise is that it moves. As your nail grows out over weeks, the dark spot travels toward the tip and eventually grows off the edge. A brown mark that stays in place or gets wider instead of migrating forward is not a bruise, and that distinction matters.
Melanonychia: Brown or Black Streaks
A vertical brown or black line running from the base of the nail to the tip is called melanonychia. It happens when pigment-producing cells in the nail matrix (the tissue under the cuticle where the nail forms) become activated and deposit melanin into the growing nail plate. Normally, these cells exist in your nails but stay dormant. Certain triggers wake them up.
Common triggers include repeated friction or trauma to the nail, pregnancy, certain medications (especially chemotherapy drugs, retinoids, and some antiretroviral medications), and vitamin B12 deficiency. Melanonychia is also more common in people with darker skin tones and can be entirely normal in that context. It affects single or multiple nails and can appear as one thin streak or several bands.
Most cases of melanonychia are benign. But because it can look identical to an early nail melanoma, any new or changing dark streak on a nail deserves evaluation by a dermatologist.
Vitamin B12 Deficiency
Low vitamin B12 levels can trigger brown or blue-black pigmentation in the nails, often appearing as dark longitudinal streaks or a reticulate (net-like) pattern. The discoloration typically starts near the cuticle and progresses outward. You might also notice darkening of the skin on the fingertips and nail beds.
The mechanism involves a chain reaction: B12 deficiency lowers levels of a protective molecule called glutathione, which normally keeps melanin production in check. Without enough glutathione, melanin synthesis ramps up, and pigment deposits form in the nail. Correcting the deficiency with supplementation typically allows the discoloration to grow out over several months as healthy, unpigmented nail replaces the affected portion.
Kidney Disease and Other Systemic Conditions
A distinctive pattern called “half-and-half nails” (also known as Lindsay nails) is associated with chronic kidney disease. The proximal half of the nail, closer to the cuticle, appears white, while the distal half near the fingertip turns red, pink, or brown. The brown band typically occupies 20% to 60% of the nail’s length, with a sharp line of demarcation between the two zones. This pattern doesn’t fade when you press on the nail.
Half-and-half nails are more common on fingernails than toenails and can also appear with liver cirrhosis and Crohn’s disease. If you notice this two-toned pattern on multiple nails, particularly alongside other symptoms like fatigue, swelling, or changes in urination, it could point to an underlying condition that needs medical attention.
When Brown Nails Could Be Melanoma
Subungual melanoma is rare, but it’s the reason dermatologists take new brown streaks on nails seriously. It typically appears as a single dark vertical line on one nail, often described as looking like someone drew a stripe with a brown or black marker. The streak usually starts narrow, less than 3 millimeters wide, and gradually widens over time, often expanding first at the base of the nail near the cuticle.
Several features distinguish melanoma from benign melanonychia:
- Widening or darkening over time. A streak that’s getting broader or darker is more concerning than one that stays the same.
- Irregular borders. Uneven edges or varying shades of brown and black within the streak suggest melanoma rather than a uniform band.
- Pigment spilling onto surrounding skin. Brown or black discoloration extending from the nail onto the cuticle or the skin around the nail is called Hutchinson’s sign and has been recognized as a warning sign for subungual melanoma since the 1880s.
- Nail damage. Splitting, cracking, deformity, or a small growth lifting the nail can accompany melanoma.
The American Academy of Dermatology recommends that any fingernail or toenail with a new or changing dark streak be examined by a dermatologist for a skin cancer check. This is especially important if you have only one affected nail, if the streak appeared without an obvious cause, or if the discoloration is evolving in any way.
How to Tell What’s Causing Your Brown Nails
The pattern gives you the biggest clue. A uniform brownish tint across several nails, especially on one hand, points to staining. A thick, crumbly nail with yellowish-brown discoloration starting at the tip suggests fungus. A dark spot that moves toward the free edge over weeks is almost certainly a bruise. A vertical streak that stays in place as the nail grows needs closer evaluation.
Multiple nails turning brown simultaneously is actually more reassuring than a single affected nail. When several nails change color at once, the cause is usually systemic (a medication, a vitamin deficiency, a medical condition) or environmental (staining, fungus). A solitary dark streak on one nail, especially a new one that’s changing, is the scenario that warrants prompt dermatologic evaluation to rule out melanoma.