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The main symptoms of acanthosis nigricans are the following:

• Hyperpigmentation: This is when certain areas of skin become darker, or more pigmented, and may turn greyish, black, or brown.
• Hyperkeratosis: This is when areas of skin become thicker and may take on a velvet-like appearance. Eventually, skin lines may become deeper and more noticeable, and wart-like growths can appear.

Acanthosis nigricans can also cause the following additional skin symptoms:

• dryness
• excessive roughness
• itching
• an unusual odor

Malignant acanthosis nigricans may cause more severe and extensive skin changes than the benign (noncancerous) form.

Some people experience these skin changes on one side of their body only. This is known as unilateral acanthosis nigricans.

Changes to the skin typically develop slowly. Occasionally, they can be present from birth, but they usually appear in childhood or adulthood. They can occur anywhere, but commonly affect the:

• anus
• armpits
• genitals
• groin
• back and sides of the neck

• Acanthosis nigricans is not a disease in itself. Rather, it is a symptom of an underlying disorder. The skin changes take place when skin cells start to reproduce too quickly.

  The following factors can cause acanthosis nigricans:

  • Insulin resistance: Acanthosis nigricans commonly affects people with obesity and insulin resistance, which is a situation where the body cannot use insulin effectively. Insulin resistance eventually leads to type 2 diabetes.
  • Hormonal changes: People with hormonal disorders, such as Addison's disease, polycystic ovary syndrome (PCOS), or hypothyroidism may develop acanthosis nigricans.
  • Genetics: Hereditary acanthosis nigricans may be present from birth, but most people develop it during childhood or later in life.
  • Medication use: Taking medications, such as birth control pills, corticosteroids, or high doses of niacin, can lead to the onset of acanthosis nigricans. Some bodybuilding supplements may also cause this skin disorder.
  • Cancer: In rare cases, malignant acanthosis nigricans can occur in people with certain types of stomach cancer, including gastric adenocarcinoma, as well as other carcinomas and lymphomas.