The short explanation
A brown or black vertical line on a nail is called longitudinal melanonychia. In people with brown skin it is fairly common and often harmless, especially when several nails have similar lines, which can be a normal variation. A single line can also come from injury, nail biting, fungal infection, some medicines or pregnancy. However, a new line on just one nail, especially if it is getting wider or darker, has uneven colour, blurred edges, or the pigment spreads onto the skin around the nail, should be checked by a dermatologist. Rarely, it can be a sign of melanoma under the nail, which is important to catch early. A dermatologist may examine it with a magnifying device and decide whether a biopsy is needed.
What depends on your situation
The parts of this that change from woman to woman. Uncertainty is stated, not hidden.
- Taking a clear photo with a ruler beside the nail every month can help track whether the line is changing.
- Nail injuries, such as catching a finger in a door, can cause dark marks that move forward as the nail grows.
- Mention any medicines you take, as some can cause nail pigmentation.
What this answer cannot determine
The boundary of online education. These need a clinician who can see you.
- This cannot tell you whether your nail line is harmless. Only a dermatologist's examination, and sometimes a biopsy, can answer that.
Questions for your clinician
Take these with you. You can add any of them to an appointment card.
- Does this nail line look harmless or concerning?
- Do I need a biopsy?
- How should I monitor it?
- Could any of my medicines cause this?
When to seek help
Clinician-approved guidance for this topic. When in doubt, being seen is never the wrong choice.
Book an appointment soon
- See a dermatologist within a couple of weeks because the line is new and on a single nail.
Raise it at a routine appointment
- Book routine follow-up checks to monitor the line as advised.
Sources
The evidence this explanation is built on.
- Skin conditions MedlinePlus (US National Library of Medicine)
- Fungal infections MedlinePlus (NIH)
- Hair loss MedlinePlus (US National Library of Medicine)
Who wrote and reviewed this
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- Author
- AskHer Editorial (AI-assisted draft) AskHer's editorial team — not an individual clinician Works in English, தமிழ், हिन्दी
- Clinical reviewer
- Nobody yet No clinician has signed this answer off, and no person on the editorial team has read it yet. That is what the “Written with AI assistance” label means. It is queued for an editor to read, and then for a clinician who works in this area. The sources it was drafted against are listed above.
- Review status
- Written with AI assistance — not yet reviewed by a person
- Last reviewed
- Not yet reviewed by a person
- Version
- 1
Correction history
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No corrections have been made since this answer was first published.
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