A black spot on your toenail is most often a bruise from trauma, even trauma you don’t remember. Blood pools under the nail and creates a dark red, purple, or black discoloration that can look alarming but is usually harmless. Less common causes include fungal infections, benign pigment changes, and, rarely, melanoma under the nail. The key to figuring out what you’re dealing with is understanding how each cause looks and behaves differently.
Bruise Under the Nail (Subungual Hematoma)
This is by far the most common explanation. Stubbing your toe, dropping something on your foot, wearing tight shoes during a long hike or run, or even repeated pressure from daily activity can rupture tiny blood vessels beneath the nail. The trapped blood creates a dark spot that typically looks like a smudge or bruise rather than a clean line. It usually appears within hours of the injury, though you may not always recall the specific moment it happened.
The telltale sign that you’re looking at a bruise: the spot moves forward as your nail grows. Toenails grow at roughly 1.6 mm per month, so tracking a spot over several weeks gives you useful information. If the dark area is slowly migrating toward the tip of your nail, it’s almost certainly old blood growing out. You may even notice the discoloration in your clippings when you trim your nails.
Most bruises under the nail resolve on their own as the nail grows out. For a big toenail, that full cycle can take 12 to 18 months, so patience is part of the process. If the hematoma is fresh (under 48 hours), painful, and large, a doctor can relieve pressure by making a small hole in the nail to drain the pooled blood. Larger hematomas with a displaced fracture underneath or visible damage to the nail fold may need more involved repair, but this is uncommon.
Fungal Infection
Most fungal nail infections produce white or yellowish discoloration, but certain species can turn a nail black. One culprit, a mold called Aspergillus niger, produces black spores that deposit in the nail’s keratin layer, creating a distinctive dark discoloration sometimes paired with a milky white base. The nail often thickens, becomes brittle, and may start separating from the nail bed.
Fungal infections develop gradually over weeks to months, not overnight. You’ll usually notice other changes alongside the color shift: crumbling edges, a distorted shape, debris accumulating under the nail, or a faint odor. Unlike a bruise, the discoloration doesn’t migrate forward cleanly with nail growth. If you suspect a fungal infection, a doctor can confirm it by taking a clipping or scraping and examining it under a microscope.
Benign Pigment Changes (Melanonychia)
Sometimes the cells that produce pigment in your nail matrix become more active, creating a brown or black vertical band that runs from the cuticle to the tip of the nail. This is called longitudinal melanonychia, and it’s extremely common in people with darker skin tones. The number and width of these bands can increase naturally with age.
These bands are typically uniform in color and width, with evenly spaced parallel lines when viewed closely. They’re painless and stable. Pregnancy, certain skin conditions like psoriasis and lichen planus, and even habitual trauma from friction (particularly on the big toe, fourth toe, and fifth toe) can trigger this kind of pigment activation. A benign mole under the nail, called a subungual lentigo or nail nevus, can also produce a single brown longitudinal band that remains consistent over time.
Medications That Darken Nails
Several drug classes can cause nail discoloration as a side effect. Chemotherapy agents, including doxorubicin, cyclophosphamide, and hydroxyurea, are well-known causes. Antimalarials like hydroxychloroquine can also darken nails. The discoloration often affects multiple nails rather than just one, which is a useful clue. If your black spots appeared after starting a new medication, that connection is worth raising with your prescriber.
When a Black Spot Could Be Melanoma
Subungual melanoma, a cancer that develops in the nail matrix, is rare but serious. It accounts for a small fraction of all melanoma cases. It’s more common in people of African, Japanese, Chinese, and Native American heritage, and it tends to occur between ages 50 and 70. The thumb, big toe, and index finger are the most frequently affected digits.
What makes melanoma look different from a bruise or benign band is the pattern. Melanoma typically appears as a dark vertical band, wider than 3 mm, with an irregular or blurred border. The band broadens over time rather than staying stable or growing out. One of the strongest clinical warning signs is called the Hutchinson sign: pigment that extends beyond the nail itself onto the surrounding skin of the cuticle or nail fold. When brown or black color bleeds into the skin around the nail, that warrants prompt evaluation.
A set of guidelines known as the ABCDEF criteria can help you assess your risk:
- Age and ancestry: 50 to 70 years old; African, Asian, or Native American descent
- Band characteristics: brown-black, wider than 3 mm, irregular borders
- Change: the band is growing wider or darker over weeks to months
- Digit: thumb, big toe, or index finger
- Extension: pigment spreading to skin around the nail (Hutchinson sign)
- Family history: melanoma in a close relative
No single criterion confirms melanoma, and the Hutchinson sign, while important, isn’t infallible. Some benign conditions can produce pigment that shows through the nail fold, mimicking the sign. But if you check several of these boxes, a biopsy is the only way to get a definitive answer.
What a Nail Biopsy Involves
If your doctor is concerned about a dark spot, they’ll likely recommend a biopsy of the nail matrix, since that’s where pigment-producing cells live. This can be done with a small punch tool or a scalpel to remove a tissue sample. In some cases, a longitudinal biopsy samples the matrix, nail bed, and nail fold all at once for the most complete picture.
The procedure is done under local anesthesia. Afterward, you’ll need to keep the toe elevated for about 48 hours to control swelling. Smaller biopsies heal on their own or with a stitch or two. The main trade-off is cosmetic: a matrix biopsy can sometimes cause the nail to regrow with a permanent split or ridge. A longitudinal biopsy carries the highest risk of scarring but provides the most diagnostic information. For something as consequential as ruling out melanoma, that trade-off is generally worth it.
How to Tell What You’re Dealing With
Start by asking yourself a few questions. Did you recently injure the toe, wear tight shoes, or do an unusually long run or hike? If so, a hematoma is the most likely explanation, and you can watch it grow out over the coming months. Is the nail thickened, crumbly, or pulling away from the bed? That points toward a fungal infection. Are multiple nails affected, and did you recently start a new medication? Drug-induced pigmentation is worth considering.
The spots that deserve closer attention are the ones that don’t fit a clear pattern: a single dark band that appeared without injury, is wider than 3 mm, has changed over time, or shows pigment spreading into the skin around the nail. These features don’t guarantee cancer, but they do justify a visit to a dermatologist who can examine the nail with a dermatoscope and decide whether a biopsy is needed.