Acanthosis nigricans is the medical term for darkened, thickened patches of skin that usually develop in the armpit and around the groin and neck.

It’s not a condition in itself, but can be a sign of an underlying health problem. These underlying conditions aren’t usually serious, although occasionally acanthosis nigricans can be sign of cancer.

These patches may occur anywhere, but are usually seen around the neck, in the armpit, around the groin and sometimes in other skin folds. Occasionally, the skin over the joints of the fingers and toes may be affected, as well as the lips, palms of the hands and soles of the feet.

The patches usually develop slowly over time. Patches that grow and spread quickly are more likely to be associated with cancer. In these cases, the mouth, tongue, throat, nose and windpipe may also be affected.

Acanthosis nigricans sometimes occurs in people who are otherwise in good health, particularly dark-skinned people of African descent.

However, in most cases it’s a sign of an underlying problem or condition such as obesity, diabetes or abnormal hormone levels.