Dark Streak Under a Nail: When to Get It Checked

A dark streak under a nail can be easy to dismiss, especially if you remember bumping a finger or toe. Sometimes it is related to trauma, pigment changes, or other non-urgent causes. But a new or changing dark band can also be associated with a nail condition that deserves prompt evaluation.

The key is not to panic, but not to ignore it either. If a streak is new, getting wider, irregular in color, or affecting the skin around the nail, it is worth getting checked by a board-certified dermatologist. Nails can be harder to read than skin, so a careful exam matters.

Quick answer

• A dark streak under a nail is often called melanonychia, which describes brown or black pigment in the nail.
• Some causes are harmless, including trauma, natural pigment variation, and certain medications or nail conditions.
• A new or changing streak, especially on one nail, should be evaluated by a dermatologist.
• Red flags include widening pigment, uneven color, nail splitting, nail lifting, or darkening of the nearby skin.
• If you’re unsure, it’s worth getting checked rather than watching it for too long.

What it is

A dark streak that runs lengthwise under a fingernail or toenail is often described as longitudinal melanonychia. In plain English, that means pigment is showing up as a stripe in the nail. The pigment may appear brown, black, or gray, and it may involve part of the nail or most of it.

That description alone does not tell you the cause. Some streaks are benign. Others can be associated with conditions that need a closer look, including melanoma of the nail unit. Because the possibilities overlap, visual clues and timing both matter.

Common causes or triggers

• Minor trauma: A bruise under the nail can look dark, especially after pressure, stubbing, or repetitive friction.
• Benign pigment changes: Some people naturally develop pigmented streaks, and this can be more common in darker skin tones.
• Moles or benign melanocytic changes: Pigment cells in the nail area can create a line or band.
• Inflammation or infection: Certain nail disorders can change nail color or create discoloration that mimics a streak.
• Medication-related changes: Some medicines can be associated with nail pigmentation.
• Melanoma of the nail unit: This is less common, but it is one of the reasons a new or changing dark streak should not be ignored.

What you can do at home

Start by noticing the pattern. Ask yourself whether the streak appeared after an obvious injury, whether it seems to be moving outward as the nail grows, and whether it is staying the same or changing. A simple photo every few weeks can help you notice change more objectively.

Avoid digging under the nail, aggressive filing, or trying to treat it with internet advice. You also do not want to cover a concerning change with polish for months without looking at it. Gentle nail care is fine, but home treatment should not delay an evaluation when warning signs are present.

Professional options

A dermatologist can evaluate the nail, the surrounding skin, and the pattern of pigment. That may include a close visual exam and, in some cases, dermoscopy to look more carefully at the nail features. If something looks suspicious, your clinician can explain whether monitoring or a biopsy is appropriate.

The goal is not to overreact to every dark mark. It is to separate harmless causes from the ones that need timely diagnosis. Nail findings can be subtle, so getting the right eyes on it can make the next step much clearer.

When to see a dermatologist

• The streak is new and you do not know why it appeared.
• It is affecting one nail only, especially if it is getting wider or darker.
• The color looks uneven, very dark, or irregular.
• The nearby skin is becoming darker.
• The nail is splitting, lifting, or developing a bump underneath.
• You expected a bruise to grow out, but it is not behaving that way.
• You have a personal or family history that makes you more cautious about skin cancer.

In short, a dark streak is not always serious, but a new or changing one should be treated as worth a professional opinion.

FAQ

Is a dark streak under a nail always melanoma?

No. There are several possible causes, and many are not cancer. Still, melanoma is one reason dermatologists take a new or changing streak seriously.

Can a bruise under the nail look like a dark streak?

Yes. Trauma can cause dark discoloration under a nail. A bruise often changes over time and may move outward as the nail grows, but not every dark mark follows a simple pattern.

What if it does not hurt?

Pain is not a reliable guide. Some concerning nail changes are painless, which is why appearance and change over time matter more than discomfort alone.

Does it matter if the skin next to the nail is getting darker too?

Yes. Darkening of the nearby skin is a reason to get evaluated promptly, especially if it is new or progressing.

Should I wait to see if it grows out?

If you are confident it is from recent trauma and it seems to be moving with nail growth, observation may feel reasonable. But if the cause is unclear, or the streak is changing, it is better to have a dermatologist examine it.

Disclaimer: This article is for educational purposes and is not medical advice. For diagnosis and personalized treatment, please book an appointment with a board-certified dermatologist.

Sources & further reading

• American Academy of Dermatology (AAD) – How to check your nails for melanoma
• DermNet – Melanonychia
• The Skin Cancer Foundation – Melanoma Warning Signs and Images