Having a line on your fingernail is called melanonychia. The lines are brown or black and can also appear on your toenails. It usually looks like a stripe that starts at the bottom of your nail bed and continues to the top.

Melanonychia may appear in one nail or several. These lines can be a natural occurrence if you have a dark complexion.

No matter what the cause may be, you should always have a doctor check for melanonychia because it can sometimes be a sign of other health issues. Melanonychia may also be called melanonychia striata or longitudinal melanonychia.

There are two broad types of melanonychia:

  • Melanocytic activation. This is an increase in the production and deposits of melanin in your nail but not an increase in pigment cells. It can be triggered by trauma, nutritional deficiency, medication, and other causes.
  • Melanocytic hyperplasia. This is an increase in the number of pigment cells in your nail bed. This growth can be benign or malignant.
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Melanonychia is black or brown discoloration of the nail. It may be longitudinal, as seen here, where it runs along the length of the nail. It can also be diffuse or transverse, where it runs along the width of the nail. Dermatology11/Shutterstock
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Here is what brown longitudinal nail discoloration may look like. Rytis Bernotas/Shutterstock
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Here is what diffuse nail discoloration may look like. Science Photo Library / Alamy Stock Photo

Melanonychia can appear on fingernails or toenails. It can also appear on more than one nail at a time.

It may look like:

  • a black or brown vertical stripe down the length of the nail
  • a black or brown discoloration stripe that is spread across the nail
  • deformity of the nail

The nails of your toes or fingers are usually translucent and not pigmented. Melanonychia is caused when the pigment cells, called melanocytes, deposit melanin — brown-colored pigment — into the nail. These deposits are usually grouped together. As your nail grows, it causes the stripe of brown or black to appear on your nail.

Melanin deposits are caused by two primary processes, which have different causes.

Melanocytic activation can be caused by:

Melanocytic hyperplasia may be caused by:

Other causes of melanonychia may include:

  • some bacteria
  • tobacco
  • hair dye
  • silver nitrate
  • henna

People of African descent are the most likely to experience melanonychia, followed by people of other races who have darker skin.

A diagnosis of melanonychia is reached after a series of exams. Your doctor will begin with a physical exam of all your fingernails and toenails. This includes looking at whether your nail is deformed, how many nails have melanonychia, as well as the color, shape, and size of your melanonychia. Your doctor will also look at your medical history to see if you have any conditions that may be causing the melanonychia.

The next step in diagnosis is a dermatoscopic examination using a specific type of microscope to get a close look at the discolored areas. Your doctor will look primarily for signs that your melanonychia may be malignant.

The signs of possible nail melanoma include:

  • discoloration which is more than 3 millimeters wide or has increased in size
  • irregular brown pigmentation
  • black or grey color with the brown
  • granular-looking pigmentation
  • deformity of the nail

Besides looking for signs of a possible melanoma, your doctor will combine findings from both the dermoscopy and the physical exam to determine the type and cause of your melanonychia.

After these two steps, your doctor may also perform a biopsy of your nail. A biopsy removes a small portion of your nail and nail tissue for examination. This step will be done in most cases of melanonychia unless there are no possible signs of cancer. A biopsy is an important step in the diagnosis of melanonychia because it will tell your doctor with certainty if it’s malignant or not.

The treatment for melanonychia varies depending on the cause. If your melanonychia has a benign cause and is noncancerous, there’s often no treatment needed.

If your melanonychia is caused by medication, your doctor may change your medication or have you stop taking it for a time, if possible. For medications that you cannot stop taking, melanonychia may just be a side effect that you will get used to.

Other treatment options depend on the cause and may include:

  • taking antibiotic or antifungal medications, if infection is the cause
  • treating the underlying disease or medical condition causing the melanonychia

If your melanonychia is malignant or cancerous, the tumor or cancerous area must be completely removed. That may mean that you’ll lose all or part of your nail. In some cases, the finger or toe that has the tumor may need to be amputated.

Possible complications of melanonychia include nail cancer, bleeding under the nail, splitting of your nail, and deformity of your nail. The nail biopsy can also cause nail deformity because it removes a portion of the nail.

The outlook for most benign melanonychia is good, and in most cases, does not require treatment. However, it usually does not go away by itself.

The outlook for malignant melanonychia is not as good. This condition requires the removal of the tumor which may also include amputation of your finger or toe. Cancer of the nail is challenging to catch in the early stages because of its similarities to the benign causes of melanonychia. Research has found that performing a biopsy on most melanonychia is the best way to get an earlier diagnosis.