Our skin often reveals more than we realize. Changes in color, texture, or unusual markings can sometimes serve as early warnings of underlying health conditions. While not every dark patch is dangerous, abnormal skin darkening—known as acanthosis nigricans—can, in rare cases, signal hidden cancers.

Acanthosis nigricans causes skin to become darker, thicker, and sometimes velvety in texture. It is more commonly linked to conditions like diabetes, obesity, or hormonal imbalances. However, in some instances, it has been associated with serious illnesses such as stomach cancer, liver cancer, or lymphoma.

Below are five areas of the body where unusual darkening should never be overlooked.

1. Neck

The neck is one of the most common areas where abnormal pigmentation appears. If your skin looks persistently “dirty” despite washing, or feels thicker than usual, this could be more than a cosmetic concern. Certain cancers release hormones and proteins that disrupt skin cell growth, leading to this kind of discoloration.

2. Armpits

While deodorant use or friction can cause darkness in the armpits, sudden, velvety, thickened patches may be a red flag. In some cases, cancer-related acanthosis nigricans shows up in hidden areas like the underarms and develops without an obvious cause.