Black Nail: Causes, Is It Dangerous, and How a Dermatologist Tells a Bruise from Subungual Melanoma
A black nail (Black Nail) is a nail plate that turns black or dark brown. It can come from a harmless bruise under the nail or from subungual melanoma, which needs prompt diagnosis. The difference lies in the "pattern", not in how dark the colour is.
Many patients come in worried that a black nail means cancer. In reality, most black nails have a non-serious cause, but a small proportion are overlooked and diagnosed late. This article explains how dermatologists actually tell them apart in the clinic.
What can cause a black nail?
Common causes, in order of how often they are seen in practice:
Bruise under the nail (Subungual Hematoma) — from a knock, a pinched finger or tight shoes. Very common in runners and people who stand for long periods.
Longitudinal pigment band (Longitudinal Melanonychia) — pigment cells at the nail root become more active, or there is a mole under the nail.
Fungal infection (Fungal Melanonychia) — some fungi produce their own pigment, causing a dark nail together with a thick, crumbly, brittle nail.
Pseudomonas bacterial infection (Chloronychia) — the nail turns greenish-black, usually after the nail lifts off the nail bed or after frequent hand soaking in water.
Medicines and systemic conditions — chemotherapy, hydroxyurea, zidovudine (AZT), minocycline, antimalarial drugs, or adrenal insufficiency (Addison's disease).
Subungual melanoma (Subungual Melanoma) — uncommon, but the one cause that must never be missed.
The first question a dermatologist asks: one nail or several?
This is the first criterion used to sort the possible causes, and you can check it yourself right away.
Several nails dark at the same time — usually a whole-body cause: medicines, a systemic condition, or natural pigmentation linked to ethnicity. The risk of cancer in this group is low.
One dark nail, especially the thumb, big toe or index finger — needs a careful assessment, because subungual melanoma usually affects a single nail and favours these digits.
Having only one affected nail does not mean it is cancer, but it does mean it needs to be examined, not just watched.
How is a bruise under the nail different from pigment?
The point that confuses people most can be separated by:
History — a bruise usually follows a clear injury and hurts at first. Pigment usually does not hurt and appears gradually.
Movement — a bruise moves outward as the nail grows and has a clearly curved upper edge. A pigment band stays attached to the nail root and keeps growing out continuously without disappearing.
Timing — fingernails grow about 3 mm per month and toenails about 1 mm per month, so a bruise on a toenail can take 9–12 months to grow out completely.
Colour — a bruise is usually dark red, purple or dark brown, and fades over time. Melanin pigment is an even brown to black, or becomes darker.
The most important caution: if the dark mark has not grown out within 2–3 months, or there was no injury at all, do not assume it is a bruise. A number of subungual melanomas are diagnosed late because the patient believed it was a bruised nail.
Besides the colour change, always look at the surface and thickness of the nail too. A dark band together with ridges or vertical grooves in the nail surface points more towards chronic inflammation or a tumour at the nail root than ordinary pigment. A thick, cloudy yellow nail with crumbly debris underneath is usually nail fungus, which can sometimes make the nail look dark enough to be mistaken for a black nail, and a thin nail that peels off in layers easily is a different group of problems altogether. If you are not sure which group your nail belongs to, see the full symptom comparison table in the nail disease guide.
Warning signs: the ABCDEF rule for subungual melanoma
Dermatologists use the ABCDEF criteria (Levit et al.) to screen for subungual melanoma:
A — Age & Ancestry: age 50–70 or older, and Asian, African or Hispanic ancestry, carry a higher risk.
B — Band: a brown to black lengthwise band wider than 3 millimetres, with blurred borders and uneven colour.
C — Change: the line changes quickly, becoming wider or darker, or does not improve despite treatment.
D — Digit: it occurs on the thumb, big toe or index finger, and affects only one nail.
E — Extension: pigment spreads onto the skin around the nail or the nail fold (Hutchinson's sign).
F — Family/personal history: a personal or family history of melanoma or atypical moles.
The more criteria that apply, the sooner you should see a dermatologist for an examination with proper instruments, rather than waiting and watching.
What is Hutchinson's sign and why does it matter?
Hutchinson's sign is when dark pigment spreads from under the nail onto the skin around the nail or the nail fold. It is one of the most specific signs of subungual melanoma.
But there is a look-alike called pseudo-Hutchinson's sign, where pigment under the nail "shows through" the thin translucent skin around the nail so that it looks like it has spread, even though it has not. It can be seen with ethnic pigment bands, with moles under the nail in children, and in Laugier-Hunziker syndrome. Telling the two apart with the naked eye is difficult and needs a magnified look with a dermoscope, which is why a single black nail should be examined by a dermatologist.
Are Asians really at higher risk of subungual melanoma?
True in proportion, but it needs to be understood correctly:
Asians develop cutaneous melanoma much less often than white people.
But when it does occur, it is usually the Acral Lentiginous Melanoma type, which arises on the palms, soles and under the nails, and it makes up a clearly higher share than in white people.
On the other hand, harmless longitudinal pigment bands (ethnic melanonychia) are naturally common in people with darker skin and in Asians as they get older, especially when several nails are affected.
In short, there is no need to panic over every brown line, but a line that meets the ABCDEF criteria should not be ignored.
Black nails in children: should we worry?
Generally much less worrying than in adults. Most pigment bands in children's nails come from a mole under the nail (nail matrix nevus), which is benign, and they are often wide and dark and may show a pseudo-Hutchinson's sign even though it is not cancer. Subungual melanoma in children is very rare.
Even so, the approach is systematic follow-up by a dermatologist (photographs, measuring the width and dermoscope examination at intervals), not an immediate biopsy and not leaving it alone.
How does a dermatologist examine a black nail?
Taking a history and examining every nail — how many nails are abnormal, any history of injury, medicines used, and underlying conditions.
Dermoscope examination (Onychoscopy) — looking at the pigment pattern: even, parallel brown lines are usually benign, while lines with uneven width, colour and spacing suggest an abnormality. It also separates a bruise (seen as round blood-red dots) from melanin.
Fungal testing (KOH / culture) — when fungus is suspected it should be confirmed before treatment, and antifungal tablets should not be started without testing.
Nail matrix biopsy (Nail Matrix Biopsy) — the final standard for diagnosis when the clinical picture and the dermoscope findings are still suspicious. It is done by an experienced dermatologist to reduce the chance of permanent nail deformity.
How is it treated?
Treatment depends on the cause, not on how dark the nail is:
Bruise under the nail — most heal by themselves as the nail grows out. If there is a lot of pain in the first 24–48 hours, the doctor may drain the blood to relieve the pressure.
Nail fungus — oral or topical antifungal medicine according to the test results, taking several months in line with the nail growth cycle.
Pseudomonas — trimming the lifted part of the nail, keeping the nail dry, together with a topical medicine.
Pigment from medicines or systemic disease — review the medicines with your doctor and treat the underlying disease. The nail colour usually fades on its own.
Uncertain mole under the nail — follow-up with photographs and dermoscope, or a biopsy.
Subungual melanoma — must be surgically removed with clear margins. Today, many cases at the in situ stage can be treated with finger-sparing surgery, and amputation is not always necessary, which is another reason why early examination matters so much.
When should you see a dermatologist right away?
One black nail with no history of injury
A dark line that gets wider, darker or uneven in colour
Pigment spreading onto the skin around the nail
A dark mark that has not grown out with the nail within 2–3 months
A black nail together with a split or lifting nail, bleeding, or a lump under the nail
Frequently asked questions
What conditions can a black nail point to?
Many, from a bruise after an injury, nail fungus, medicine side effects and adrenal insufficiency to subungual melanoma. Sorting them out relies mainly on how many nails are affected, any history of injury, and the appearance of the dark mark.
What nail colour carries a risk of cancer?
A lengthwise brown to black band wider than 3 millimetres, with uneven colour and blurred borders, on a single nail, that is enlarging quickly or has pigment spreading onto the skin around the nail (Hutchinson's sign), needs prompt examination.
Is a black nail from running or tight shoes dangerous?
Most are a bruise under the nail, which is harmless and heals by itself as the nail grows out. But if the dark mark has not grown out with the nail within 2–3 months, a dermatologist should examine it, because subungual melanoma is often mistaken for a nail bruised by shoes.
What causes a dark nail root?
It may be pigment made by melanocytes at the nail root, a mole under the nail, or chronic friction. If pigment spreads onto the skin around the nail, it can be a sign of melanoma and should be examined with a dermoscope.
Can frequent manicures cause a black nail?
Yes. Friction, picking at the nails and repeated injury at the nail root can stimulate pigment cells to make brown lines. But when only one nail is affected and it looks abnormal, cancer must always be ruled out.
Consult a skin specialist
A black nail cannot be judged by the naked eye alone. Examining the nail with a dermoscope is a simple, painless procedure that takes little time but can change the whole course of care. The dermatologists at Siam Dermatology are ready to examine and follow you up systematically.
Written by Dr. Patara Voravutinon, Dermatologist · Last reviewed 12 July 2026
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