[3] Acanthosis nigricans appears as dark brown-black, poorly defined, velvety patches of skin, typically affecting the face, neck, underarms, genitals, groin, elbows, knees, anus, umbilicus and nasal crease.

[1] It typically occurs in individuals younger than age 40, is associated with insulin resistance, type 2 diabetes, obesity or endocrinopathies, such as hypothyroidism, acromegaly, polycystic ovary syndrome or Cushing's disease, and may be genetically inherited.

Higher than normal insulin levels in the blood stream cause the growth of darkened skin over certain areas of the body.

No skin treatment will cure it, but acanthosis nigricans may lighten up and possibly disappear by treating the root cause, insulin resistance, but it can take months or years to do so.

[12] Acanthosis nigricans has been linked to the use of nicotinic acid,[9] glucocorticoid use, combined oral contraceptive pills, and growth hormone therapy.

[7]: 506  Malignancy-associated acanthosis nigricans is usually rapid in onset and may be accompanied by skin tags, multiple seborrheic keratoses, or tripe palms.

[8]: 676 Acral acanthotic anomaly refers to a variant of acanthosis nigricans limited to the elbows, knees, knuckles, and dorsal surfaces of the feet, in the absence of any other findings, in otherwise healthy individuals.

[19] Acanthosis nigricans is caused by increased activation of growth factor receptor proteins, usually due to endocrine dysfunction.

Selenium sulfide topical 2 percent applied in thin layer to dry skin ten minutes prior to bathing may clear symptoms.