7 causes of Dark Line Under Toenail You Shouldn’t Ignore

A dark line under toenail is one of the nail findings that most consistently generates patient anxiety — and for good reason, because it is also the nail finding where getting the clinical assessment right matters most. Unlike yellow nails or white spots, a dark line under the toenail carries a non-negligible probability of representing something that needs professional evaluation.

Most dark lines under toenails are benign. The majority are caused by minor trauma, natural pigmentation from melanocyte activation, or medication effects. But a small percentage represent subungual melanoma — a form of skin cancer that develops in the nail matrix and produces a pigmented streak through the nail plate. And the consequences of missing a subungual melanoma diagnosis are severe enough that the approach to any dark line under toenail should involve serious consideration rather than automatic reassurance.

This guide explains what causes a dark line under toenail, how to distinguish concerning from benign presentations, what the Hutchinson’s sign means and why it matters, and the specific features that should prompt you to see a professional without delay.

Current image: Dark Line Under Toenail — When to See a Doctor

What a Dark Line Under Toenail Actually Is

A dark line under toenail most commonly appears as a vertical streak — running from the base of the nail (near the cuticle) toward the nail tip. The medical term for this is longitudinal melanonychia — melanonychia meaning black nail, longitudinal meaning lengthwise.

There are two distinct ways a dark line under toenail can form:

Within the nail plate itself:
The nail matrix contains melanocytes — pigment-producing cells. When these cells become activated for any reason, they deposit melanin pigment into the nail plate cells as they are being produced. Since the matrix produces nail continuously and the nail plate grows forward from the matrix, a band of melanin-containing cells in the nail plate produces a dark streak running the length of the nail from base to tip.

In the nail bed beneath the plate:
Blood or dark-colored material can accumulate in the space between the nail bed and the nail plate, producing a dark line under toenail that is beneath — rather than within — the nail plate.

Understanding which mechanism is producing the dark line helps determine whether it is benign or concerning. Nail plate-origin streaks (from melanocyte activation) need assessment. Nail bed-origin discoloration (from trauma and bleeding) is usually benign.

7 Causes of a Dark Line Under Toenail

1. Subungual Hematoma — The Most Common Benign Cause

A subungual hematoma — blood collected between the nail plate and nail bed following trauma — is the most common cause of a dark line under toenail in otherwise healthy adults.

After a direct impact, internal nail trauma, or prolonged repetitive nail-to-shoe contact during running, capillaries in the nail bed rupture. Blood collects in the subungual space. As the clot forms and the blood breaks down over days to weeks, the color progresses from bright red to dark red, purple, brown, and eventually brownish-black.

What makes a hematoma identifiable:

Clear trauma history — specific injury event, new shoes, increased running mileage, dropping something on the foot
The dark area appears in the body of the nail or the tip — not starting from the cuticle
The discoloration moves forward toward the nail tip as the nail grows — over months, you can watch it progress toward the tip, where it eventually trims off
No spreading of pigment to the surrounding skin

The critical test — does it move?
A hematoma is physically within the subungual space, not in the nail plate cells. As the nail grows forward, the hematoma-containing area (between the nail and nail bed) moves with the nail. Over 3 to 6 months, the dark area should be visibly progressing toward the nail tip. This movement is the most reliable indicator of a benign traumatic cause.

2. Longitudinal Melanonychia From Melanocyte Activation

When the nail matrix melanocytes activate and deposit melanin into the growing nail plate, a longitudinal streak of pigment appears as a dark line under toenail. This is melanocyte activation in the nail matrix producing a streak within the nail plate — not blood in the subungual space.

This can occur from:

Benign causes of melanocyte activation:

Normal variation in individuals with darker skin tones (African, Asian, South Asian, Mediterranean, Hispanic backgrounds)
Aging — melanocyte activity in the nail matrix often increases with age
Inflammatory nail conditions — psoriasis, lichen planus, and nail trauma can activate nearby melanocytes without being malignant
Medications (see below)
Friction or repetitive mechanical stimulation of the nail matrix

What benign melanonychia looks like:

A relatively narrow streak — typically less than 2 to 3 mm wide
Uniform color throughout — the same shade of brown or dark throughout the streak
Stable width over months and years — not visibly widening
Does not move toward the nail tip in the way a hematoma does — the streak stays in position because it is being produced continuously by the matrix
No pigment visible on the surrounding skin

The important nuance:
Unlike a hematoma, melanonychia-type dark lines do not move toward the nail tip — they are continuously produced by the activated matrix and maintain a consistent position from base to tip. This is why melanonychia and hematoma behave differently over time.

3. Subungual Melanoma — The Critical Differential

Subungual melanoma is a form of acral lentiginous melanoma — the most common melanoma subtype in non-white populations, though it affects all skin types. It develops from melanocytes in the nail matrix that become malignant and produce an increasingly abnormal pigmented streak.

Subungual melanoma accounts for approximately 1 to 3 percent of all melanomas in white populations but 15 to 35 percent of melanomas in darker-skinned populations. Despite being relatively rare, it carries a significantly worse prognosis than other melanoma types — partly because it is frequently diagnosed at a later stage due to delayed recognition and evaluation.

Features that raise concern for subungual melanoma:

Width: A benign melanonychia streak is typically narrow and consistent. A stripe wider than approximately 3 mm, or one that is visibly widening over months, is a concerning feature.

Color irregularity: Benign streaks tend to have fairly uniform color from edge to edge — the same shade throughout. Melanoma streaks often have multiple shades — darker at the edges, lighter in the middle, or multiple distinct colors within the same streak.

Hutchinson’s sign — the most critical warning sign:
Hutchinson’s sign describes the extension of pigment from the nail plate onto the surrounding periungual skin — the nail fold (the skin immediately adjacent to the nail), the cuticle, or the skin on the toe pad beneath the nail tip. When dark pigment crosses from the nail plate onto the surrounding skin, this is a highly significant clinical warning sign for subungual melanoma. This requires urgent professional evaluation — measured in days, not weeks.

Nail plate destruction:
The presence of nail splitting, lifting, crumbling, or structural distortion alongside a pigmented streak is a concerning feature — benign melanonychia does not typically damage the nail plate structure.

Single nail involvement:
Benign melanonychia from medication or normal pigmentation tends to affect multiple nails. Subungual melanoma almost always affects a single nail.

New onset in older adults:
A completely new dark line under toenail appearing after age 50, without a clear trauma history and in a previously unaffected nail, carries higher clinical suspicion than the same finding in a young person where benign activation is more common.

4. Benign Nail Nevus (Mole Under the Nail)

The nail matrix can develop a benign nevus — a collection of pigmented cells similar to a skin mole — that produces a pigmented streak through continuous deposition of melanin into the growing nail plate.

A matrix nevus produces a dark line under toenail that:

Is typically narrow and relatively uniform in color
Has been present and stable for months to years
Does not widen over time
Has no surrounding skin pigmentation
Is more common in children and young adults

Professional dermoscopic assessment can characterize the pigment pattern and help determine whether biopsy is needed.

5. Medications and Systemic Causes

Several medications produce dark nail streaks by activating nail matrix melanocytes. Key medications include:

Hydroxyurea — used for myeloproliferative disorders; produces longitudinal and transverse melanonychia
Bleomycin — chemotherapy agent with well-documented nail pigmentation effects
Doxorubicin and other anthracyclines — chemotherapy with nail matrix effects
Antimalarials (hydroxychloroquine, chloroquine) — produce nail pigmentation as a recognized side effect
Zidovudine (AZT) — antiretroviral with documented nail pigmentation effect

Medication-related dark lines under toenails typically:

Affect multiple nails simultaneously
Onset correlates with the start of a specific medication
May involve horizontal banding as well as longitudinal streaks
Do not spread to surrounding skin

6. Systemic Conditions Activating Nail Melanocytes

Several systemic conditions can activate nail matrix melanocytes, producing benign but clinically notable dark lines:

Addison’s disease (adrenal insufficiency) — excess ACTH production stimulates melanocytes throughout the body including nail matrices
Peutz-Jeghers syndrome — inherited condition with mucosal and nail pigmentation
HIV/AIDS — associated with nail melanonychia independent of medication effects
Chronic kidney disease — dialysis patients sometimes develop nail pigmentation

7. Fungal Infection With Pigmented Organisms

Certain fungal organisms produce dark pigment as part of their biology. Some mold species — including Trichophyton violaceum — produce melanin and can create dark discoloration including streak patterns. However, fungal-related dark lines are almost always accompanied by structural nail changes — thickening, crumbling, surface irregularity — rather than isolated clean streaks.

The Hutchinson’s Sign: Understanding the Most Important Warning Sign

The Hutchinson’s sign deserves expanded discussion because it is the single most clinically important feature distinguishing a dark line under toenail that needs immediate evaluation from one that can be monitored.

What Hutchinson’s sign is:
Pigmentation spreading from the nail plate onto the surrounding periungual skin — specifically:

The proximal nail fold (the skin immediately adjacent to the nail at the base/cuticle)
The lateral nail folds (the skin along the sides of the nail)
The hyponychium (the skin visible at the nail tip beneath the nail plate)
The toe pad or adjacent finger skin

Why it matters:
In subungual melanoma, the malignant melanocytes eventually invade beyond the nail matrix and nail plate into the surrounding skin. This produces pigmentation visible on the periungual skin around the nail — the Hutchinson’s sign.

When Hutchinson’s sign is present, the probability of subungual melanoma is dramatically elevated. This is a reason to seek evaluation within days, not weeks.

What is not Hutchinson’s sign:
Normal pigmentation of the nail fold skin in darker-skinned individuals who have benign ethnic melanonychia. In some people with dark skin tones and benign nail pigmentation, mild pigmentation of the nail fold may be a normal variant. This can cause difficulty in clinical assessment, which is one reason dermoscopy and specialist evaluation are important in ambiguous cases.

A Practical Assessment Framework

When you notice a dark line under toenail, these questions help assess the situation:

1. Was there a clear trauma event?
A specific impact, repetitive running trauma, or new tight footwear preceding the streak makes hematoma more likely. No trauma history increases the index of suspicion for melanocyte-related causes.

2. Is the dark area moving toward the nail tip?
Check monthly. Take photographs. A hematoma moves forward with the nail — progressively reaching the tip over 3 to 6 months. A melanonychia streak does not move — it is reproduced at the same position continuously by the matrix.

3. How wide is the streak?
Less than 2 mm and stable — more reassuring. Wider than 3 mm or visibly widening — seek evaluation.

4. Is the color uniform?
Consistent brown throughout — more reassuring. Multiple shades, irregular darkness, or variable color — seek evaluation.

5. Is any pigment visible on the skin surrounding the nail?
Any periungual skin pigmentation in connection with the nail streak — seek urgent evaluation (days, not weeks).

6. Is the nail plate intact?
Normal nail structure — more reassuring. Splitting, crumbling, or distortion alongside the streak — seek evaluation.

7. Is only one nail affected?
Multiple nails affected — more consistent with systemic or medication cause (generally more benign). Single nail with a new streak — evaluate.

Comparing Dark Line Under Toenail Causes at a Glance

Feature	Subungual Hematoma	Benign Melanonychia	Subungual Melanoma
Trauma history	Usually present	Not required	Not required
Movement with nail growth	Yes — moves toward tip	No — maintained by matrix	No — may expand toward base
Color	Dark red/brown/black, uniform	Brown, relatively uniform	Multiple shades, irregular
Width	Corresponds to trauma area	Narrow, stable	Often wider, widening
Hutchinson’s sign	Absent	Absent	May be present
Nail structure	Intact	Intact	May show distortion
Number of nails	Single	Can be multiple	Almost always single
Action needed	Monitor for movement	Professional evaluation	Urgent evaluation

When to Seek Professional Evaluation

Seek evaluation within days (urgent) if:

Any pigmentation is visible on the skin surrounding the nail — the Hutchinson’s sign
The dark line is widening visibly over weeks
The nail plate is splitting, crumbling, or distorting alongside the streak
The line appeared suddenly in a single nail with no trauma history in an adult over 50

Schedule a routine evaluation within 1 to 2 weeks if:

A new dark line appeared without clear trauma explanation
You are uncertain whether the streak is moving
The streak is wider than approximately 3 mm
The color appears irregular or multi-toned

Monitor at home with monthly photographs if:

There was a clear trauma event that preceded the streak
The dark area is visibly moving toward the nail tip over months
The streak is narrow, uniform in color, and the nail structure is intact
The surrounding skin shows no pigmentation

See a podiatrist or dermatologist for evaluation if:

You have been monitoring and the streak is not moving toward the tip after 3 months
Any new features develop (widening, skin involvement, nail distortion)
You are simply unsure and would like professional assessment — which is always a reasonable choice

Frequently Asked Questions

Is a dark line under toenail always dangerous?

No. The majority of dark lines under toenails are caused by benign conditions — subungual hematoma from trauma, benign melanonychia from normal pigmentation, or medication effects. Subungual melanoma is a real but uncommon cause. Specific features — width, color irregularity, Hutchinson’s sign — distinguish concerning from benign presentations.

What is the Hutchinson’s sign and why is it important?

Hutchinson’s sign is the spread of pigment from the nail onto the surrounding periungual skin — the nail fold, cuticle, or toe pad. Its presence is a major clinical warning sign for subungual melanoma and requires urgent professional evaluation within days.

How do I know if my dark nail line is moving?

Take a clear, well-lit photograph of the nail including a reference point (like the cuticle) today. Repeat monthly for 3 to 4 months and compare. A hematoma-related dark line will visibly progress toward the nail tip over months. A melanonychia streak maintains its position because it is produced continuously by the matrix.

Does subungual melanoma hurt?

Subungual melanoma is usually painless in its early stages — which contributes to delayed diagnosis. Pain develops when the melanoma invades surrounding tissue or when nail plate destruction occurs in advanced disease.

Can a dermatologist diagnose subungual melanoma without a biopsy?

Dermoscopy — a specialized magnifying tool with polarized light — allows experienced dermatologists to characterize pigment patterns in the nail in detail, which significantly improves assessment accuracy over visual examination alone. However, when clinical features suggest melanoma, nail matrix biopsy is the definitive diagnostic step.

Should I be more concerned about a dark line if I have darker skin?

People with darker skin tones have higher baseline rates of benign melanonychia — so a dark line under toenail is more commonly benign in this population. However, subungual melanoma also occurs in darker-skinned populations and is associated with worse outcomes due to later diagnosis. A new, changing, or atypical dark line under toenail warrants evaluation in any patient regardless of skin tone.

Summary

A dark line under toenail is most commonly either a subungual hematoma from trauma — which moves forward with the nail — or benign longitudinal melanonychia from normal melanocyte activation — which stays in position but is stable and narrow. Both of these benign causes are far more common than subungual melanoma.

However, subungual melanoma is the cause that cannot be missed. The specific features that distinguish it from benign causes are: a streak wider than approximately 3 mm, irregular or multiple-colored pigmentation, visible widening over weeks to months, single nail involvement, and most critically — the Hutchinson’s sign, where pigment spreads from the nail onto the surrounding skin.

When Hutchinson’s sign is present, or when any of the other concerning features are identified, professional evaluation measured in days rather than weeks is the appropriate response. For stable, narrow, uniform streaks with a clear trauma history and visible forward movement — monthly monitoring with photographs is reasonable, with professional evaluation if any features change.

Your nails are visible and assessable daily. Using the framework in this guide to monitor your own dark line under toenail systematically — and knowing exactly when to escalate to professional evaluation — gives you the best possible chance of catching any serious finding at the stage when it is most treatable.

Dark Line Under Toenail — When to See a Doctor

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