Acanthosis is a thickening of the outermost layer of skin, specifically the middle portion called the prickle cell layer. On its own, it’s a histological term describing what’s happening under a microscope. But most people encounter it through acanthosis nigricans, a visible skin condition where dark, velvety patches develop in body folds and creases. These patches are usually a signal from your body that something metabolic is going on underneath the surface.
What Acanthosis Looks Like
The hallmark of acanthosis nigricans is patches of skin that appear light brown to black, feel velvety or thicker than surrounding skin, and typically show up in areas where skin folds against itself. The back of the neck is the most recognized spot, but it also commonly appears in the armpits, groin, under the breasts, on the knuckles, and in the creases of the elbows and knees.
The patches can look like a stain or smudge that you might try to scrub off, but they don’t wash away. Some people also develop small skin tags in or around the affected areas. The patches may itch, and in some cases they produce a noticeable odor. The condition tends to develop gradually, so you might not notice it right away.
Why It Happens: The Insulin Connection
In most cases, acanthosis nigricans is driven by too much insulin circulating in the blood. When insulin levels stay high over time, the excess insulin interferes with a growth-regulating system in the body. It triggers the liver to produce more of a growth factor called IGF-1 while suppressing the protein that normally keeps IGF-1 in check. The result is that skin cells in vulnerable areas start multiplying faster than normal, causing the visible thickening and darkening.
This makes acanthosis nigricans one of the earliest visible clues that your body is becoming resistant to insulin. The skin changes can appear before blood sugar levels rise enough to be flagged on routine lab work. The longer insulin stays elevated and the higher it climbs, the more extensive and severe the skin patches tend to become. This is why the condition is closely tied to obesity, type 2 diabetes, and metabolic syndrome, all of which involve chronic insulin resistance.
Conditions Linked to Acanthosis Nigricans
The condition most commonly appears in people under 40 and is associated with a range of metabolic and hormonal conditions:
- Insulin resistance and type 2 diabetes, the most frequent underlying cause
- Obesity, which drives insulin resistance even before diabetes develops
- Polycystic ovary syndrome (PCOS), where excess androgens and insulin resistance overlap
- Hypothyroidism, which can slow metabolism and promote weight gain
- Cushing disease and Addison disease, both involving hormonal imbalances
- Acromegaly, a condition of excess growth hormone
People of Hispanic ethnicity appear to have a higher risk, with one large study finding roughly 2.5 times the odds of developing acanthosis nigricans compared to non-Hispanic individuals. The condition affects all skin tones but is often more visible on darker skin.
Medications That Can Trigger It
Certain drugs can cause acanthosis nigricans independently of metabolic disease. Nicotinic acid (a form of niacin used to treat cholesterol) and injected insulin are the two most commonly reported culprits. Systemic corticosteroids and some other medications have also been linked to the condition. If patches appear shortly after starting a new medication, that timing is worth noting and bringing up with your prescriber.
When It Signals Something More Serious
In rare cases, acanthosis nigricans is a paraneoplastic syndrome, meaning it appears as a reaction to an internal cancer rather than a metabolic problem. This malignant form has distinct features: it comes on suddenly, spreads extensively, and progresses quickly. It looks similar to the metabolic form but tends to be more dramatic in scope.
About 73% of malignant acanthosis nigricans cases are linked to cancers inside the abdomen, with stomach cancer (gastric adenocarcinoma) being the most common, accounting for 56% to 61% of cases. Cancers of the liver, uterus, breast, lung, pancreas, and colon have also been associated. The skin changes can appear before, at the same time as, or after a cancer diagnosis. A rapid, widespread onset in someone without obesity or known metabolic disease is the key red flag that distinguishes this form.
How It’s Diagnosed
Doctors typically recognize acanthosis nigricans on sight. The combination of dark, velvety patches in characteristic locations is distinctive enough that a biopsy is rarely needed. The more important step is figuring out what’s driving it. That usually means blood work to check fasting insulin levels, blood sugar, and markers of metabolic health. Depending on the clinical picture, thyroid function tests or screening for hormonal conditions like PCOS may follow. If the presentation suggests a malignant cause, imaging and further workup for internal cancers become the priority.
Treatment Options
Treating acanthosis nigricans effectively means addressing whatever is causing it. For the vast majority of people, that means tackling insulin resistance. Weight loss through dietary changes and exercise is the most impactful intervention, as even modest reductions in body weight can lower circulating insulin and allow the skin to gradually normalize. Medications that improve insulin sensitivity, like metformin, are also effective and are often prescribed alongside lifestyle changes.
For the cosmetic appearance of the patches themselves, topical retinoids are considered a first-line option. These work by speeding up skin cell turnover and can help reduce the thickness and darkening over time. Creams containing urea or salicylic acid have also been used with mixed results. These topical approaches address the symptom rather than the cause, so they work best in combination with treatment of the underlying condition. When the metabolic driver is controlled, the skin patches often fade on their own, though this can take months.