Dark fingernails usually result from something harmless: natural pigmentation related to skin tone, a bruise from injury, or a side effect of medication. In people with darker skin, pigmented bands in the nails are extremely common, appearing in up to 77% of individuals. Still, dark discoloration can sometimes signal a fungal infection, a nutritional deficiency, or, rarely, skin cancer under the nail. The cause matters, so understanding what different types of darkening look like helps you figure out what you’re dealing with.
Natural Pigmentation Is the Most Common Cause
The single most frequent reason for dark lines or bands running lengthwise down a fingernail is simply your natural skin tone. These bands, which can range from light brown to nearly black, are produced by pigment cells in the nail matrix (the tissue under your cuticle where the nail grows). About 1.4% of white Europeans have visible nail bands, compared to 77% of people with the darkest skin tones. In one study of pigmented nail streaks, nearly 69% were attributed to racial or ethnic pigmentation, with no underlying disease at all.
These bands often appear on multiple nails, tend to be relatively uniform in color, and stay stable over time. They can show up in childhood or develop later in life. If you have darker skin and notice even brown-black lines on several nails, this is overwhelmingly likely to be normal.
Bruises Under the Nail
A dark spot or patch under a fingernail after you’ve jammed it in a door, hit it with a hammer, or done something repetitive with your hands is almost certainly trapped blood. These bruises typically appear within a day or two of the injury, though sometimes you won’t remember the specific moment it happened, especially after vigorous activity.
The key feature of a nail bruise is that it moves. As your nail grows out, the dark spot drifts toward the tip over weeks to months. A small horizontal groove often appears near the cuticle about two months after the injury. If you take a photo and compare it a few months later, you’ll see the discoloration has shifted forward and the area closest to the cuticle has cleared. This forward drift is the most reliable way to confirm you’re looking at old blood rather than something more concerning.
Fungal Infections
Fungal nail infections are often associated with yellow or white nails, but certain fungi produce dark pigment that turns nails brown, gray, green, or black. The most common culprits are dermatophytes, which account for over 75% of fungal nail infections, along with molds like Aspergillus and Alternaria species. In the study of pigmented nail streaks mentioned earlier, about 7% were caused by fungal infections.
Fungal darkening usually comes with other visible changes: thickening of the nail, crumbling at the edges, scaling underneath, and sometimes the nail lifting away from the nail bed. If your nail is dark and also looks distorted, chalky, or fragile, a fungal infection is a strong possibility. These infections don’t resolve on their own and typically require antifungal treatment.
Vitamin B12 and Nutritional Deficiencies
Low vitamin B12 can darken the nails, skin, and mucous membranes. The mechanism involves increased activity of the enzyme that produces melanin, leading to excess pigment deposited in the nails and surrounding skin. Hands and feet are particularly affected. Other nutritional causes include severe protein deficiency and low vitamin D, though these are less common in developed countries.
The good news is that nutritional darkening is reversible. After B12 levels are corrected through supplementation, the hyperpigmentation typically fades within 6 to 12 weeks. If dark nails appeared alongside fatigue, tingling in the hands or feet, or mouth sores, a B12 deficiency is worth investigating with a simple blood test.
Medications and Chemotherapy
Dozens of medications can darken the nails as a side effect. Chemotherapy is the most well-known trigger, with nail changes typically appearing one to two months after treatment begins. Some chemotherapy drugs produce a distinctive pattern: alternating bands of dark and normal nail corresponding to treatment cycles, creating a striped appearance as the nail grows.
Specific drug classes known to cause darkening include alkylating agents, platinum-based drugs, and certain targeted therapies. Antimalarials, some antibiotics, and antiretroviral medications can also cause it outside the context of cancer treatment. If your nails darkened after starting a new medication, the timing is usually the biggest clue. This type of discoloration is cosmetic and generally resolves after the medication is stopped, though it can take months for the affected nail to fully grow out.
Addison’s Disease and Other Systemic Conditions
Certain medical conditions cause darkening across multiple nails, often alongside skin changes elsewhere on the body. Addison’s disease, a condition where the adrenal glands don’t produce enough hormones, is a classic example. Nail pigmentation can actually be the first and only sign of Addison’s disease, sometimes appearing years before other symptoms like fatigue, weight loss, and low blood pressure develop.
Kidney failure is another cause. When the kidneys can’t properly filter waste, pigments like melanin and other compounds build up and deposit in the nails. Connective tissue diseases like lupus and scleroderma can also trigger dark nail bands. In all of these cases, the nail changes affect multiple fingers and are accompanied by other health symptoms, even if those symptoms are subtle at first.
When Dark Nails Could Be Melanoma
Melanoma under the nail is rare, making up roughly 1% to 3.5% of all melanomas in Western populations, but it’s the cause that matters most to catch early. It typically starts as a single dark streak on one nail, running from the base to the tip like a line drawn with a marker. The streak may be brown, black, or irregularly shaded.
Several features distinguish a concerning streak from a harmless one:
- Single nail involvement. Melanoma almost always affects one digit, not multiple nails.
- Widening over time. The band starts narrow (under 3 millimeters) but gradually grows wider, especially at the base near the cuticle.
- Irregular color. Rather than a uniform shade, the streak contains varying tones of brown and black.
- Skin discoloration around the nail. Called the Hutchinson sign, this is pigment that spreads from the nail onto the surrounding cuticle or fingertip skin. It’s one of the most important warning signs.
- Adult onset. A new solitary streak appearing for the first time in an adult, particularly someone middle-aged or older with fair skin, warrants closer evaluation.
Over time, a melanoma may also cause the nail to lift, bleed, develop a nodule, or become ulcerated. Unlike a bruise, the discoloration does not drift forward with nail growth because the pigment is continuously produced by cancerous cells in the nail matrix.
How to Tell What You’re Looking At
The most useful first step is simply paying attention to how many nails are affected and whether the discoloration changes over time. Dark bands on multiple nails that have been stable for years are almost certainly benign, especially if you have darker skin. A bruise will migrate toward the tip as your nail grows. A fungal infection will come with nail thickening and crumbling.
A single dark streak on one nail that’s new, widening, irregularly colored, or accompanied by pigment spreading onto the surrounding skin is the pattern that needs professional evaluation. Taking a clear photo and comparing it after two to three months is one of the simplest and most effective ways to track whether a streak is changing. Dermatologists use a dermatoscope, essentially a magnifying lens with a built-in light, to examine the pigment structure more closely and can biopsy the nail matrix if needed.