Black nails most often result from trauma, like slamming a finger in a door or dropping something on your toe. But the discoloration can also signal a fungal infection, a medication side effect, or in rare cases, a type of skin cancer that develops under the nail. The cause usually depends on whether the color appeared suddenly or gradually, affects one nail or several, and whether you remember injuring it.
Trauma and Bruising Under the Nail
The most common reason for a nail turning black is a subungual hematoma, which is simply blood trapped between the nail and the nail bed. This happens when sudden force damages the tiny blood vessels underneath, such as stubbing your toe, dropping a heavy object on your foot, or catching your finger in a door. The pooled blood has nowhere to go, so it sits under the nail and appears dark red, purple, or black.
Small hematomas that cover less than half the nail and don’t cause much pain typically resolve on their own. The discolored portion gradually grows out as the nail grows, which can take several months for fingernails and up to a year for toenails. Larger hematomas that cover more than half the nail bed and cause throbbing pressure may need a simple drainage procedure called trephination, where a small hole is made in the nail to release the trapped blood. The nail will still look discolored for weeks or longer afterward, but the dark area moves outward over time until it’s eventually trimmed away.
Runners and hikers frequently develop black toenails from repetitive microtrauma. Shoes that are too tight or too loose allow the toes to bang against the front of the shoe with every step, and over long distances this causes the same bruising effect. If you regularly get black toenails after exercise, it’s worth checking your shoe fit.
Fungal and Bacterial Infections
Nail fungus is caused by dermatophytes, yeasts, and molds that thrive in warm, moist environments. While fungal infections more commonly turn nails yellow or white, certain organisms can produce dark brown or black discoloration. The nail often becomes thick, crumbly, and distorted in shape as the infection progresses.
Bacterial infections, particularly from Pseudomonas, tend to cause green or black discoloration. These infections often start when moisture gets trapped under an already damaged or lifted nail. Unlike a bruise, an infected nail typically develops its color change slowly and may have an unpleasant odor. The surrounding skin can become red or swollen.
Melanoma Under the Nail
Subungual melanoma is a rare but serious form of skin cancer that develops in the nail matrix, the tissue at the base of the nail where new nail cells form. It typically appears as a dark brown or black longitudinal streak running from the base of the nail toward the tip. Unlike a bruise, which stays in place and grows out, a melanoma streak persists and often widens over time.
Dermatologists use a set of warning signs sometimes called the ABCDEF criteria to evaluate suspicious nail pigmentation. The key features include: the streak affecting a single digit (most often the thumb or big toe), a band wider than 3 millimeters, color that’s irregular or changes over time, and pigment that extends beyond the nail onto the surrounding skin fold. That last sign, called Hutchinson’s sign, is particularly concerning because it suggests the melanoma is spreading outward from the nail bed.
Subungual melanoma is diagnosed through a biopsy. It’s more common in older adults and can occur in people of all skin tones. Because early detection dramatically improves outcomes, any new dark streak on a single nail that wasn’t caused by an obvious injury deserves a dermatologist’s evaluation, especially if it’s changing in width or color.
Normal Pigmentation Variations
Not every dark line on a nail is dangerous. Longitudinal melanonychia, a pigmented vertical streak running along the nail, is a common and completely benign finding in people with darker skin tones. It’s especially prevalent among people of African, Asian, and Hispanic descent. These streaks are caused by increased melanin production in the nail matrix and can appear on one or multiple nails.
The distinction between a benign pigment band and something worrisome comes down to pattern. Benign bands tend to be uniform in color, stable over time, and often present on multiple nails. A streak that appears suddenly on a single nail, changes width, or develops uneven coloring warrants closer inspection.
Medications That Darken Nails
Certain drugs can trigger melanin production in the nail matrix, leading to brown or black bands or overall darkening. Chemotherapy drugs are among the most common culprits, particularly hydroxyurea. Antiretroviral medications like zidovudine and some treatments used for skin conditions involving UV light therapy can produce the same effect. The discoloration usually appears as horizontal or vertical bands across one or more nails and typically fades after the medication is stopped, though it takes months for the affected nail to grow out completely.
If your nails started darkening after beginning a new medication, the timing is an important clue. This type of discoloration is a cosmetic side effect rather than a sign of nail damage, but it’s worth mentioning to your prescriber so they can confirm the cause.
Underlying Health Conditions
In some cases, black or darkened nails reflect something happening elsewhere in the body. Chronic kidney disease can increase melanin production, causing the far end of the nail bed to turn brown or dark. An overactive thyroid has been linked to brown nail discoloration as well. Excessive fluoride intake over long periods can turn nails brown or black.
These systemic causes typically affect multiple nails rather than just one, which helps distinguish them from trauma or melanoma. The nail changes also tend to appear alongside other symptoms of the underlying condition, such as fatigue, skin changes, or unexplained weight shifts. Darkened nails alone rarely lead to a diagnosis of systemic disease, but they can be one piece of a larger clinical picture.
How to Tell the Difference
A few practical questions can help you narrow down the cause. Did the discoloration appear suddenly after an injury? It’s almost certainly a bruise. Is it developing slowly on a single nail with no trauma history? That needs professional evaluation. Are multiple nails affected, or did the change coincide with starting a medication? Systemic causes or drug side effects are more likely.
The growth test is also useful for bruises. Because a hematoma is trapped blood, it moves toward the tip of the nail as the nail grows. A melanoma streak stays anchored at the base because the pigment is continuously produced by abnormal cells in the nail matrix. Watching whether the dark area migrates or stays fixed over a few weeks can be telling, though this observation shouldn’t replace a professional assessment for any streak that looks suspicious.