Black nails most often result from an injury that traps blood beneath the nail, but they can also signal a fungal infection, a medication side effect, or in rare cases, a type of skin cancer. The cause usually depends on whether one nail is affected or several, whether the color appeared suddenly or gradually, and whether you recently hurt your finger or toe.
Injury and Trapped Blood
The most common reason for a single nail turning black is a subungual hematoma, which is blood pooling underneath the nail plate. Slamming your finger in a door, dropping something on your toe, or wearing shoes that are too tight can all injure the tiny blood vessels in your nail bed. Because the nail plate is firmly attached to the tissue beneath it, the blood has nowhere to drain. It pools, creates pressure, and causes a dark red, purple, or black discoloration along with throbbing pain.
A small hematoma often resolves on its own as the nail grows out. Fingernails grow at roughly 3.5 mm per month, so a full replacement takes about six months. Toenails grow at less than half that speed, around 1.6 mm per month, meaning a blackened toenail can take nine months or longer to look normal again. If the pressure underneath is intense, a doctor can relieve it by making a small hole in the nail plate to let the blood drain, which usually provides immediate pain relief.
Fungal Infections
Most people picture thick, yellow nails when they think of fungal infections, but certain fungi produce dark pigments that turn nails brown or black. These are caused by a group of pigment-producing molds rather than the typical fungi behind ordinary nail infections. The infection tends to develop slowly, affecting one or two nails at first, and may cause the nail to become thick, crumbly, or slightly separated from the nail bed.
Yeast species and common dermatophytes (the usual culprits in athlete’s foot) can occasionally darken nails as well. Black fungal nail infections are more common in warm, humid climates and in people who go barefoot frequently. They require antifungal treatment, which can take months because the medication needs to work as the nail slowly grows out.
Medications That Darken Nails
Several medications cause nail hyperpigmentation as a side effect. Chemotherapy drugs are the most well-known offenders. Certain chemo agents can trigger dark bands or overall darkening across multiple nails, sometimes on both hands and feet. If you’re undergoing cancer treatment and notice your nails changing color, it’s a recognized side effect rather than a new problem. The discoloration typically fades after treatment ends, though it takes a full nail growth cycle to clear completely.
Antimalarial drugs and some antibiotics can produce similar changes. In most cases the darkening is harmless and reversible, but it’s worth mentioning to your care team so they can rule out other causes.
Underlying Health Conditions
Nails can act as a window into what’s happening elsewhere in the body. Several systemic conditions cause darkened nails:
- Kidney disease: Increased melanin production in chronic kidney failure can turn the far end of the nail bed brown, sometimes creating a distinctive “half-and-half” pattern where the base of the nail is white and the tip is dark.
- Thyroid disorders: An overactive thyroid can cause brown discoloration of the nail plate.
- Adrenal insufficiency: Conditions that affect hormone regulation, like Addison’s disease, can trigger widespread darkening of the skin and nails.
- Nutritional deficiencies: Low levels of B12 and folate have been linked to darkened nail pigmentation.
When a health condition is the cause, multiple nails are usually affected rather than just one. Treating the underlying condition often stops the discoloration from progressing, though existing dark areas still need to grow out.
External Staining
Sometimes the explanation is simply something you touched. Hair dye is a frequent culprit, especially dark permanent formulas that stain the nail surface and surrounding skin. Nicotine from cigarettes tends to yellow nails, but heavy use can push the staining toward brown or black. Certain industrial chemicals, potassium permanganate (used in some wound treatments), and silver nitrate can also leave dark deposits on the nail surface.
The key difference with external staining is that the color sits on or near the surface of the nail rather than underneath it. Scrubbing with acetone or waiting for the top layer of nail to grow out usually resolves it.
When Black Nails Could Be Melanoma
Subungual melanoma is a rare but serious form of skin cancer that starts in the nail matrix or nail bed. It typically appears as a dark brown or black streak running lengthwise along the nail. It accounts for a larger share of melanoma diagnoses in people with darker skin tones. Peak incidence falls between the ages of 40 and 70, and the thumb and big toe are the digits most commonly involved.
Dermatologists look for several warning signs. A pigmented band wider than 3 mm with irregular or blurred borders is more concerning than a thin, uniform line. A streak that changes in width, darkness, or shape over weeks to months deserves prompt evaluation. One of the most important clues is called Hutchinson’s sign: pigment that spreads from the nail onto the surrounding skin of the cuticle or side folds. In one study of confirmed nail melanomas, this sign was present in about 83% of cases. It’s worth noting that roughly one-third of benign nail streaks can mimic this sign through the transparent nail fold, so the width, continuity, and pattern of the pigment all matter in distinguishing the two.
A personal or family history of melanoma raises the level of concern. If you have a single dark streak on one nail that you can’t trace to an injury, and especially if it’s new, widening, or accompanied by pigment spreading onto the skin around the nail, getting it examined by a dermatologist is the right move. Early-stage nail melanoma is treatable, but delays in diagnosis are common because people assume the dark mark is a bruise or a harmless streak.
How to Tell the Difference
A few practical clues help narrow down what’s going on. A bruise usually follows a clear injury, starts out painful, and moves forward as the nail grows. A fungal infection develops gradually, often alongside nail thickening or crumbling, and may affect neighboring nails over time. Medication-related changes tend to appear on multiple nails at once, shortly after starting a new drug. Melanoma almost always affects a single nail, appears as a lengthwise streak rather than a blob, and progresses slowly without an obvious trigger.
If a black nail doesn’t hurt, wasn’t caused by an injury you remember, and hasn’t started growing out after a couple of months, those are all reasons to have it looked at rather than waiting.