Acanthosis nigricans !

Dark, thick, velvety skin in folds such as the neck and armpits; usually a sign of insulin resistance and, rarely, of an internal cancer.

Updated 2026-11-01

Structures involved

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Why it happens

What it is

A skin change with symmetric, hyperpigmented, velvety and thickened plaques in skin folds. It is not a disease in itself but a cutaneous marker: most often of insulin resistance (obesity, type 2 diabetes, polycystic ovary syndrome) and, in a rare sudden and extensive form, of internal malignancy.

Who gets it

Common and rising with obesity. It is more frequent in people with darker skin, which includes many Colombian and Latin American patients, and is increasingly seen in children and adolescents with excess weight, where it is a useful screening sign for insulin resistance and future type 2 diabetes. The malignant form is rare and appears in middle-aged or older adults, often without obesity.

Causes

Insulin resistance and hyperinsulinaemia (by far the most common): obesity, type 2 diabetes, metabolic syndrome, polycystic ovary syndrome
Other endocrine disorders: Cushing syndrome, acromegaly, hypothyroidism
Drugs: systemic corticosteroids, nicotinic acid, oestrogens and hormonal contraceptives, injected insulin at injection sites
Malignancy: mainly gastric adenocarcinoma and other abdominal adenocarcinomas
Familial and syndromic forms (rare)

Risk factors

Obesity, especially central obesity
Family history of type 2 diabetes
Darker skin type
Polycystic ovary syndrome
Sedentary lifestyle

Pathophysiology

High insulin levels activate insulin-like growth factor 1 receptors on keratinocytes and dermal fibroblasts, which stimulates their proliferation and causes papillomatosis and hyperkeratosis. The dark colour comes mostly from thickened keratin rather than more melanin. In the malignant form, tumour-derived growth factors (such as TGF-α acting on the epidermal growth factor receptor) produce the same effect much more quickly.

Classification

Benign (obesity / insulin resistance-related, the most common)
Syndromic and familial
Drug-induced
Malignant (paraneoplastic)
Acral and unilateral variants

Symptoms and signs

Brown to grey-black, velvety, thickened skin
Back and sides of the neck, armpits, groin, under the breasts, elbows and knuckles
Skin tags (acrochordons) in the same areas
Usually no symptoms; occasionally mild itch or odour
Signs of the cause: obesity, hirsutism, irregular periods, high blood pressure

Diagnosis

Clinical: Clinical, by the typical velvety dark plaques in folds. The key step is to look for the cause: weight, waist circumference, blood pressure and signs of hyperandrogenism.
Tests: Fasting glucose and HbA1c (or an oral glucose tolerance test), lipid profile, and insulin or other insulin-resistance markers when useful; hormonal tests if PCOS or Cushing syndrome is suspected. Biopsy only in atypical cases.
Imaging: Not needed in the benign form. In suspected malignant acanthosis nigricans: upper endoscopy and cross-sectional imaging (CT of chest, abdomen and pelvis).

Differential diagnosis

Confluent and reticulated papillomatosis
Post-inflammatory hyperpigmentation and intertrigo
Pellagra (photo-exposed, not flexural)
Addison disease hyperpigmentation
Poor hygiene ('terra firma-forme' dermatosis), which wipes off with alcohol

Goal of treatment

Treat the underlying cause (above all, insulin resistance), prevent progression to type 2 diabetes and, in the malignant form, find and treat the cancer; cosmetic improvement is secondary.

Treatment

Lifestyle: Weight loss, healthy diet and regular physical activity reduce insulin levels and often lighten the skin over months.
Medical: Insulin sensitizers (metformin) when indicated for prediabetes, diabetes or PCOS; stop causative drugs where possible.
Medical: Cosmetic topical therapy: keratolytics (salicylic acid, urea), topical retinoids or vitamin D analogues.
Procedure: Selected lasers or chemical peels for persistent cosmetic concern; in malignant forms, treatment of the tumour is the only effective option.

Complications

Progression to type 2 diabetes and cardiovascular disease (from the underlying cause)
Psychosocial impact and stigma
Missed diagnosis of an internal malignancy in the rare paraneoplastic form

Prognosis

The benign form is harmless and often fades as weight and insulin levels fall. The malignant form follows the course of the cancer, which is often advanced at diagnosis.

Prevention

Prevent and treat childhood and adult obesity
Screen people with acanthosis nigricans for prediabetes and diabetes

Test yourself

Think of the answer, then tap the question to see it.

What does acanthosis nigricans most often indicate?

Insulin resistance (obesity, type 2 diabetes, metabolic syndrome, polycystic ovary syndrome).

Which cancer is classically linked to sudden, extensive acanthosis nigricans?

Gastric adenocarcinoma.

Where are the typical lesions?

Back and sides of the neck, armpits, groin and other skin folds.

What is the main treatment of the common form?

Treating the cause: weight loss and lifestyle change, with insulin sensitizers when indicated.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: Acanthosis nigricans (MedlinePlus, US National Library of Medicine, 2026); Acanthosis Nigricans: Causes, Diagnosis, and Treatment (DermNet, 2026).

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Representative model, for educational purposes only.

Skin highlighted on the Durgical 3D model