Dark Underarms and Neck: Acanthosis Nigricans Explained

Dark, thick, velvety skin on the back of the neck, underarms, groin or inner thighs is most often acanthosis nigricans, a skin sign linked to insulin resistance, excess weight, PCOS and type 2 diabetes. It is not caused by dirt and does not scrub away, but it often lightens when the underlying cause is treated, with topical treatment from a dermatologist helping the skin texture and colour.

In India, where insulin resistance and diabetes are very common, acanthosis nigricans is something dermatologists see every day, in adults, teenagers and even children. It is worth understanding because the skin is often giving an early signal about metabolic health.

What does acanthosis nigricans look like?

Typical features include:

• Darker patches of skin, brown to almost black.
• Thickened, velvety texture, unlike the flat darkening of a mark.
• Location in body folds: back and sides of the neck, underarms, groin, under the breasts, inner thighs, elbows, knees and knuckles.
• Skin tags in the same areas.
• Sometimes mild itch or odour.

It develops slowly, so many people do not notice until someone points it out.

What causes acanthosis nigricans?

The most common cause is insulin resistance. When the body does not respond well to insulin, insulin levels rise. High insulin stimulates skin cells to grow faster, which thickens and darkens the skin in folds.

Common associations include:

Cause	Notes
Insulin resistance and excess weight	The most common cause in India
Prediabetes and type 2 diabetes	AN can appear years before diabetes is diagnosed
PCOS	Often alongside acne, excess hair and irregular periods
Family history	Some families are predisposed
Hormonal conditions	Thyroid or adrenal disorders
Medicines	Some hormones, steroids and other drugs
Cancer (rare)	Sudden, widespread AN in an adult who is not overweight needs prompt evaluation

If you also have adult acne or irregular periods, read our guide on hormonal acne and PCOS.

Are all dark underarms acanthosis nigricans?

No. Several things cause darker underarms, and the treatment differs.

Type of darkening	Clues
Acanthosis nigricans	Velvety, thick skin; neck and other folds often involved too
Post inflammatory hyperpigmentation	Flat darkening after shaving, waxing, deodorant irritation or rashes
Friction darkening	Tight clothing or skin rubbing in heat
Fungal or bacterial infections	Red, brown or scaly patches, sometimes itchy, in folds
Natural skin tone	Folds are often slightly darker than the rest of the body

Flat dark marks from irritation are covered in our guide on post inflammatory hyperpigmentation. Fungal infections in folds are very common in Indian summers; see our guide on ringworm and tinea.

What tests might I need?

Because acanthosis nigricans is often a sign of something inside, your doctor may suggest:

• Fasting blood sugar and HbA1c to check for prediabetes or diabetes.
• Insulin levels or other measures of insulin resistance, in some cases.
• Lipid profile and blood pressure check.
• Thyroid tests if symptoms suggest it.
• Hormone tests in women with signs of PCOS.
• Further evaluation if the darkening appeared suddenly, spread quickly, or involves the palms, lips or mouth.

How is acanthosis nigricans treated?

Treat the cause first

This is the most important step. When insulin resistance improves, the skin often lightens and softens gradually. Steps that may help include:

• Gradual, sustained weight loss if you are overweight. Even a modest loss can improve insulin sensitivity.
• Regular physical activity, such as brisk walking, cycling or strength training.
• Reducing refined carbohydrates and sugary drinks, and choosing whole grains, pulses, vegetables and protein.
• Treating diabetes, PCOS or thyroid disease as advised by your doctor.
• Reviewing medicines with your doctor if one may be contributing. Do not stop prescribed medicines on your own.

Topical treatments your dermatologist may consider

• Retinoids to thin and smooth the thickened skin.
• Keratolytic creams such as those with urea, lactic acid or salicylic acid.
• Other lightening agents as part of a supervised plan.
• Topical antibacterial washes or creams if there is odour or irritation.

Procedures

In selected cases, chemical peels or lasers may be used to improve texture and colour. These need careful planning on darker skin to avoid irritation and further pigmentation.

What should I avoid?

• Scrubbing hard with loofahs, pumice or rough cloths. This irritates the skin and makes darkening worse.
• Lemon, baking soda, bleach or household cleaners. These can burn the skin.
• Steroid or fairness creams. They do not treat the cause and can thin the skin in folds, where steroid side effects appear quickly. Read about the risks in our article on topical steroid misuse.
• Harsh deodorants or hair removal creams on irritated skin.

How long does it take to improve?

Change is slow. With weight loss and better blood sugar, many people see gradual lightening and softening over months. Topical treatments can add improvement, but results vary and the patches may not clear completely. The health benefits of addressing insulin resistance are important regardless of the cosmetic result.

What about children and teenagers?

Acanthosis nigricans in children and teenagers, often on the neck, is increasingly common and is usually linked to excess weight and insulin resistance. It is a helpful early warning. Parents should avoid scrubbing or blaming hygiene, and instead consult a doctor about healthy eating, activity and appropriate tests.

Does weight loss really make a difference to the skin?

For many people, yes. Insulin resistance improves with weight loss and regular exercise, and as insulin levels come down, the stimulus for the skin to thicken and darken reduces. Research and clinical experience show that acanthosis nigricans often softens first, then gradually lightens, over several months of sustained change.

Some practical, affordable steps that suit Indian routines:

• Walk after meals, even for 10 to 15 minutes, which helps blood sugar.
• Swap sugary drinks, including packaged juices and sweet chai several times a day, for water, buttermilk or unsweetened tea.
• Build meals around dal, vegetables, eggs, paneer or fish, with smaller portions of rice, roti and refined flour.
• Include strength training twice a week, which improves how muscles use glucose.
• Sleep well, because poor sleep worsens insulin resistance.

These changes also lower the long term risk of diabetes and heart disease, which matters far more than skin colour alone.

When to see a dermatologist

Please consult a dermatologist or doctor if:

• You notice dark, velvety skin on your neck, underarms or groin.
• The patches appeared suddenly, are spreading quickly, or involve your palms, lips or mouth.
• You also have irregular periods, acne, excess hair or weight gain.
• You have a family history of diabetes and have not had your blood sugar checked.
• The skin is itchy, smelly, painful or not improving.
• You are unsure whether the darkening is acanthosis nigricans or something else.

Acanthosis nigricans is your skin offering an early clue about your health. If you would like help understanding your skin and the next steps, you can book an online consultation with Dr. Maria Ansari.