Having a dark line on your fingernails or toenails is called melanonychia. The lines are brown or black and usually look like a stripe that begins at the bottom of your nail bed and continues to the top.

Melanonychia may appear in one nail or several. These lines can occur naturally if you have a dark complexion.

No matter the cause, have a doctor check for melanonychia because the lines may be a sign of other health issues. Melanonychia is also 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. Causes include trauma, nutritional deficiency, and medication.
  • Melanocytic hyperplasia: This is an increase in 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 in fingernails or toenails. It can also occur in more than one nail.

It may look like:

  • a black or brown vertical stripe down the length of the nail
  • a black or brown discoloration stripe spread across the nail
  • a structural irregularity of the nail

The nails of your toes or fingers are usually translucent and nonpigmented. Melanonychia occurs when the pigment cells, called melanocytes, deposit melanin — brown-colored pigment — into the nail. These deposits are usually grouped together. As your nail grows, a stripe of brown or black appears.

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

Causes of melanocytic activation can include:

Causes of melanocytic hyperplasia can include:

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 with dark skin.

A diagnosis of melanonychia is reached after a series of exams. A healthcare professional may also review your medical history for conditions that might cause melanonychia.

They will likely examine your fingernails and toenails. This includes looking at:

  • whether your nail has a structural irregularity
  • how many nails have melanonychia
  • the color, shape, and size of your melanonychia

A doctor may do a dermatoscopic exam using a specific type of microscope to closely examine the discolored areas. They typically look for signs that your melanonychia may be malignant.

The signs of possible nail melanoma include:

The doctor may also perform a biopsy of your nail. Doctors typically recommend this step in most cases of melanonychia unless there are no possible signs of cancer.

The treatment for melanonychia depends on the cause. If your melanonychia is noncancerous, you often do not need treatment.

If your melanonychia is caused by medication, a doctor may change your medication or have you stop taking it for a time, if possible. For medications that you cannot stop, melanonychia may 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 disease or medical condition causing the melanonychia

If your melanonychia is malignant or cancerous, the tumor or cancerous area must be removed. That may mean 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
  • structural irregularity of your nail

The nail biopsy can also cause a structural irregularity because it removes a portion of the nail.

What does a dark line on your nail mean?

A dark line on your nail can result from nail injury, infection, or cancer, among other causes. A doctor can perform exams to determine whether melanonychia is malignant.

What causes dark stripes on nails?

Dark stripes on nails typically appear due to increased deposits of melanin, which gives skin its color. Melanonychia can develop due to many causes, including nail cancer, trauma, and other health conditions.

What deficiency causes black lines on nails?

Deficiencies in vitamin B12, vitamin D, and protein can cause dark lines in nails. Other causes can include cancer, too much iron, and other health conditions.

What does stage 1 nail melanoma look like?

A dark vertical line on the nail can indicate nail melanoma. It can also occur with other benign causes and requires diagnosis by a healthcare professional.

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

Malignant melanonychia requires the removal of the tumor, which may also include amputation of the 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 from 2017 has found that a biopsy of most melanonychia is the best way to get an early diagnosis.