Acanthosis Nigricans: Why the Skin on the Neck and Armpits Turns Dark and Velvety, What It Says About Insulin, Why Scrubbing and Bleaching Fail, and What It Means Before Cosmetic Surgery

The patient is 34. For about three years, the back of their neck has looked like a tide line that will not wash off. They have tried a loofah, a pumice stone, baking soda, a lemon juice recipe from the internet, and a bleaching cream bought online. The skin is now darker, sore, and still velvety. They have come in asking about a laser, and possibly about liposuction under the chin while they are at it. Nobody has yet told them the name of what is on their neck, which is acanthosis nigricans, or suggested that the most useful first test may be a blood draw.

That is the unusual thing about acanthosis nigricans. It is a skin finding that people bring to a cosmetic office, and the most important information it carries is usually not cosmetic. In most adults it points toward insulin resistance. Occasionally it points to a medication. Very rarely, it points to an internal cancer. The evidence for treating the skin itself is thin and short term, so what follows labels small studies and single case reports as what they are.

This piece covers what the patch actually is, what drives it, the workup worth having, what fades it, and how it changes planning for cosmetic surgery. It sits alongside the earlier pieces on blood sugar and cosmetic surgery and benign skin growths and skin tags, which often turn up in the same patient.

What acanthosis nigricans is, and why it is not dirt or a tan

The short answer: acanthosis nigricans is a thickening of the skin, usually in body folds, that looks dark because the surface layers have built up and folded, so it does not wash off and is not primarily a melanin problem.

The typical patch is brown to gray-black, soft, and velvety, with poorly defined edges and exaggerated skin lines. The usual places are:

• The back and sides of the neck, the most common site and the one most often noticed by someone else first.
• The armpits, where it is often mistaken for shaving irritation or deodorant staining.
• The groin and inner thighs, along with the skin under the breasts and around the belly button.
• The knuckles, elbows, and knees, which can be harder to tell apart from ordinary darkening.

The name is more descriptive than the condition deserves. Acanthosis refers to thickening of the epidermis, and nigricans means blackening. Under the microscope, the main findings are hyperkeratosis (too much surface keratin) and papillomatosis (small finger-like folds in the skin). The darkness comes largely from that thickened, folded surface rather than from a dramatic increase in pigment. That single detail explains much of the frustration in the opening story. A bleaching agent aimed at melanin is usually aimed at the wrong target, and a scrub is an injury to skin that is already reacting.

How common it is depends heavily on who is counted. In a 1989 Texas study, every sixth and eighth grader in the Galveston public schools had their neck examined during a state health survey, and 7.1 percent of 1,412 children had acanthosis nigricans. A published review cites a prevalence as high as 19.4 percent among US primary care patients. Rates differ widely by ancestry: another review reports roughly 40 percent of Native American teenagers, 13 percent of African American, 6 percent of Hispanic, and under 1 percent of non-Hispanic white children aged 10 to 19 with clinically apparent disease. Part of that is biology and metabolic risk, and part may be how easily the change is seen in a given skin tone.

Several conditions can look similar:

• Terra firma-forme dermatosis is a harmless buildup of keratin and debris that looks like dirt. It classically wipes away with 70 percent isopropyl alcohol, which a clinician can test in the office. Published case series suggest that the simple wipe can spare patients unnecessary workups.
• Post-inflammatory hyperpigmentation follows friction, rashes, or shaving, and it is flat rather than velvety. The razor bumps and ingrown hairs piece covers one common source.
• Fungal and bacterial rashes such as tinea versicolor and erythrasma can discolor body folds and respond to their own treatments.
• Hidradenitis suppurativa can darken the armpit and groin, but it comes with painful lumps, drainage, and scarring. The hidradenitis piece covers that separately.

Self-diagnosis is not the goal here. A dermatologist or primary care clinician can usually make the call by looking, and a biopsy is needed only in unusual cases.

What is driving it: insulin resistance, PCOS, medications, and the rare cancer warning

The short answer: insulin resistance is by far the most common cause of acanthosis nigricans, followed by hormonal conditions and certain medications, while cancer-related acanthosis nigricans is rare but has a recognizable pattern that should prompt an urgent medical workup.

The leading explanation for the common form runs like this. When the body becomes resistant to insulin, the pancreas produces more of it. At high levels, insulin is thought to bind to insulin-like growth factor 1 receptors on keratinocytes and fibroblasts, the cells that build the skin, and that signal tells them to grow. The explanation is not fully settled. Some researchers question whether insulin levels in typical patients with obesity get high enough to do this alone, and fibroblast growth factor and epidermal growth factor signaling are also implicated. But the clinical link is strong, and published work reports that severity tracks with fasting insulin.

The American Diabetes Association lists acanthosis nigricans among the conditions associated with insulin resistance when it describes which young people with overweight or obesity should be screened for type 2 diabetes. That is an unusually practical status for a skin finding. It is a visible signal that may appear before anyone has checked a glucose level.

The recognized causes fall into a few groups:

• Insulin resistance and obesity: The most common setting, including prediabetes and type 2 diabetes. It can also occur in people who do not look overweight, so body size alone does not settle the question.
• Polycystic ovary syndrome and other hormonal conditions: PCOS is a common companion, and HAIR-AN syndrome (hyperandrogenism, insulin resistance, and acanthosis nigricans) is a recognized cluster. Irregular periods, acne, or new coarse facial hair next to a dark neck should go on the list for the clinician. Cushing syndrome and thyroid disease are also on the differential.
• Medications: Nicotinic acid (high-dose niacin) is the classic example. Systemic corticosteroids, oral contraceptives, testosterone, and some HIV protease inhibitors have also been linked. The supplement aisle counts here, since high-dose niacin can be bought without a prescription, which is one reason the supplement stop list matters.
• Repeated insulin injections: A localized patch can form at an injection site that is used over and over. It is uncommon, and rotating sites is the main response. It matters because absorption of insulin into thickened skin may be unreliable.
• Inherited forms: Some families carry it, and certain gene changes, including in fibroblast growth factor receptors, can cause it without insulin resistance.

Then there is the rare form. Malignant acanthosis nigricans is a paraneoplastic sign, a skin change produced by a tumor elsewhere. Small series put gastric adenocarcinoma at roughly 55 to 69 percent of cases, and those numbers come from case collections rather than population data. In one summary, about 61 percent of cases were found around the time of the cancer diagnosis and about 20 percent before it.

What separates this form from the common one is the story, and these features deserve prompt medical attention rather than a cosmetic appointment:

• Sudden onset or rapid spread, especially in an adult with no weight change or known insulin resistance.
• Widespread involvement, including unusual sites.
• Velvety changes in the mouth or on the lips.
• Thickened, ridged palms, a sign known as tripe palms.
• A sudden eruption of many seborrheic keratoses or skin tags.
• Unintended weight loss, new difficulty swallowing, or persistent abdominal pain.

Most people with a dark neck do not have cancer, and this list is not a reason to panic. It is a reason not to let an unusual presentation get filed under skin care.

The workup worth having before anyone treats the skin

The short answer: a person with new acanthosis nigricans should have a medical history, a medication review, and basic metabolic testing, because the finding is more useful as a prompt to look for insulin resistance than as a target for a device.

For most adults, this is a primary care visit, and it is modest in scope. A reasonable conversation covers when the patch appeared, how quickly it changed, family history of diabetes, menstrual history, and every medication and supplement in use. Testing commonly includes a hemoglobin A1c or fasting glucose, sometimes with a lipid panel, blood pressure, and liver enzymes. In people with signs of excess androgens, hormone testing may be added, and the clinician decides what fits.

Two details are worth knowing. First, the American Diabetes Association uses an A1c of 5.7 to 6.4 percent as the prediabetes range and 6.5 percent or higher as the diabetes threshold, but a normal A1c does not prove that insulin levels are normal. The pancreas can hold glucose steady for years by producing more insulin, which is part of what makes the skin finding interesting. Second, fasting insulin testing is not standardized the way A1c is, so clinicians differ on whether to order it.

The same finding also overlaps with other surgical concerns. Metabolic syndrome, fatty liver, and sleep apnea tend to travel together, and the earlier piece on fatty liver disease and cosmetic surgery goes through one of those links. Finding any of them before an elective operation is good news, because it is the moment when a problem can be managed rather than discovered under anesthesia.

"A dark, velvety neck is one of the few cosmetic complaints that can send a patient to the right blood test years before a diagnosis would have found them. The patch deserves treatment. The reason for the patch deserves it first."

What actually fades acanthosis nigricans: creams, peels, lasers, and medication

The short answer: acanthosis nigricans usually improves most when its cause is treated, topical retinoids have the strongest support for the skin itself, and peels and lasers have only small studies behind them, so results are often partial and can relapse.

Treating the cause comes first. Reviews consistently state that the common form tends to regress when the underlying disorder is treated, which usually means weight reduction and better insulin sensitivity, or changing a culprit medication where that is safe. That is slow, on a timeline of many months, and fading can be incomplete. Newer weight-loss drugs may help through the same mechanism, but we found no good trials measuring this skin outcome, and the GLP-1 piece covers the surgical cautions that come with them.

For the skin itself, the evidence looks like this:

• Topical tretinoin: A 2024 systematic review of 19 clinical trials in Archives of Dermatological Research gave it the strongest recommendation of the options reviewed. In one small study of 0.05 percent tretinoin in 30 patients, 80 percent were reported to clear by 16 weeks, but relapse came within about four weeks of stopping. Tretinoin can irritate folds, and it should be avoided in pregnancy.
• Adapalene and urea: The same review gave these a moderate endorsement. Urea creams soften and thin the keratin layer, and a 2025 Frontiers in Medicine review of seven randomized trials (268 patients in total) found benefit, with the caveat that every trial lasted only about two months.
• Chemical peels: That 2025 review found a 15 percent trichloroacetic acid peel more effective than a 35 percent glycolic acid peel at eight weeks. A pilot study of six women reported mixed results, with burning and some hyperpigmentation as side effects. The laser versus chemical peel piece covers how those tools differ.
• Fractional CO2 laser: The 2024 review gave this a moderate endorsement based on limited trials. It is a controlled injury to reactive skin, with downtime, and the general tradeoffs are covered in the ablative and non-ablative laser piece.
• Long-pulsed alexandrite laser: Often quoted, rarely examined. The claim of greater than 95 percent improvement comes from a single case report in one patient's armpits after seven treatments. It is a hypothesis, not a track record.
• Oral retinoids: Isotretinoin and acitretin have been used in case reports, with relapse after stopping, and they carry real side effects. They are rarely justified for this, and the isotretinoin piece explains why they also complicate later procedures.
• Calcipotriol: A vitamin D analog with case-level support only.

Metformin deserves its own note because people often ask for it by name. It treats insulin resistance, and the results for the skin are mixed. One trial of 40 patients taking metformin for three months, compared with 20 controls, reported improvement on the neck and armpits but not the knuckles, fingers, or elbows. A 12-week open-label pilot comparing metformin with rosiglitazone found no change in neck lesion severity, with only modest improvement in texture. A separate small randomized trial of seven patients found no effect on severity. If a clinician prescribes metformin for prediabetes or PCOS, any skin benefit is a welcome bonus. It is not a skin treatment with proven efficacy.

Skin tone changes the risk calculation for every procedure on this list. Acanthosis nigricans is more common in people with deeper skin, and those are also the patients at higher risk of post-inflammatory hyperpigmentation from peels and lasers, which can leave the area darker than it started. The deeper skin tones piece covers why test spots and conservative settings matter. The sun spot laser piece is a useful reminder that devices marketed for pigment are not automatically suited to a thickening disorder.

Some things are worth stopping now. Loofahs, pumice, baking soda, lemon juice, and bleach are all friction or chemical injury in a fold. Unregulated skin-lightening products bought online can be worse: some have been found to contain mercury or undisclosed steroids, and long-term steroid use thins the skin. Hydroquinone used under medical supervision has a place in melasma, as the melasma piece explains, but it is not a standard solution here, since melanin is not the main problem.

Acanthosis nigricans and cosmetic surgery: liposuction, neck lifts, and pre-op planning

The short answer: acanthosis nigricans does not by itself rule out cosmetic surgery, but neither liposuction nor skin removal treats it, and the insulin resistance behind it may matter more to safety and healing than the patch does.

Start with the appealing idea that removing fat will fix the metabolism. The strongest evidence against it is a 2004 study in the New England Journal of Medicine led by Samuel Klein. Researchers studied 15 women with obesity, eight with normal glucose tolerance and seven with type 2 diabetes, before and 10 to 12 weeks after large-volume abdominal liposuction. The group with normal glucose tolerance lost about 9 kilograms of fat. Insulin sensitivity in muscle, liver, and fat did not significantly change, and neither did blood pressure, glucose, insulin, lipids, or inflammatory markers. The study was small and short, and it does not mean liposuction is pointless. It means liposuction is a contour operation, a point made in the piece on whether fat comes back after liposuction. Do not expect it to fade a dark neck through metabolic change. Weight loss through diet, activity, medication, or bariatric surgery works differently from having fat suctioned out.

A few procedure-specific points follow from that:

• Chin and neck: Submental liposuction and a neck lift reshape the area under the jaw but do not remove the skin at the back and sides of the neck, where the patch usually sits. It will still be there after surgery, and it may be more noticeable once attention is drawn to the neck.
• Abdomen and folds: A panniculectomy can remove a fold of affected skin and reduce friction, and patches on the removed skin go with it. That is a side effect of the operation, not a reason to have it, and new patches can form if the cause persists.
• Inner thighs: Inner thigh lift incisions sit in the groin crease, close to a common site for acanthosis nigricans. Warm, moist, friction-prone skin is already a difficult place to heal, so the surgeon should examine it directly and discuss scar placement.
• Laser hair removal in the armpit or groin: Darker skin in the treatment zone absorbs more energy, which raises the risk of burns and pigment change. The provider should see the area first, and settings may need adjusting. The laser hair removal piece covers why.

We did not find good studies showing that incisions through acanthosis nigricans itself heal differently. Surgeons will generally treat it as thickened skin, not diseased skin, and it is the underlying condition that carries the known risks. Poorly controlled blood sugar is associated with higher rates of infection and wound separation. Surgeons often set an A1c cutoff before elective surgery, and the exact number varies, as the earlier blood sugar piece explains. High BMI has its own limits, covered in the piece on BMI limits for cosmetic surgery.

For that reason, a visible patch should change the pre-op conversation. A surgeon who notices it should ask about diabetes risk, order or request a recent A1c as part of preoperative testing, and be willing to delay an elective operation until the result is known. A patient who has never been tested should not wait for that to happen. Also report PCOS, metformin, GLP-1 drugs, high-dose niacin, steroids, and testosterone, since each can matter around surgery in its own way. If the office does not ask, or treats the patch as a laser add-on, a second consultation is a reasonable step.

The honest summary

Acanthosis nigricans is thickened, velvety skin in the folds of the neck, armpits, and groin, and it is not caused by poor washing. Most of the time it is a visible marker of insulin resistance, sometimes alongside PCOS, and sometimes it comes from a medication such as high-dose niacin or from repeated insulin injections. Rarely, a sudden, widespread pattern with mouth or palm changes signals an internal cancer, usually of the stomach, and that needs a prompt medical evaluation.

The useful order of operations is medical first and cosmetic second. Get a diagnosis, get the basic metabolic tests, review the medication list, and stop scrubbing. If there is insulin resistance, treating it offers the most plausible route to lasting improvement, though it is slow and may be partial. Topical tretinoin has the strongest support among skin treatments, urea and peels have short-term data, and the laser literature is too small to justify confident promises, particularly in deeper skin tones where pigment changes are a real risk.

Cosmetic surgery remains possible for many people with this finding, but it does not treat the condition. Large-volume liposuction did not improve insulin sensitivity in the 2004 New England Journal of Medicine study, and a neck lift will not take away a dark band at the nape. What the patch can do is improve a surgical plan, by flagging a metabolic problem that is far easier to deal with before the operation than after it. If a dark neck is what finally gets someone to a blood test, it has already done something useful.