Acanthosis nigricans is a common skin condition that causes patches of thicker skin that are usually darker than the nearby skin. They are found most often on the neck or the underarm (axilla). But they also can occur on other areas of the body.

Acanthosis nigricans may be caused by different things. These include being very overweight, having insulin resistance or diabetes, or taking some medicines. In rare cases, it occurs when a person has cancer in another part of the body, like the stomach.

The patches usually don't cause any symptoms or need treatment. Some people choose to treat it if it is having an effect on their life. When possible, treatment of the underlying cause works best. Sometimes a medicine is prescribed that can be put on the patches to help them go away.