Most dark underarms and dark necks are caused by friction, hair removal or sun. One kind is different. The skin is darker and also thicker, with a soft, velvety feel, and it appears in several folds at once. Doctors call it acanthosis nigricans, and it matters because it is usually a sign of something happening inside the body.
This post describes what it is and what to do about it. It is not a substitute for seeing a doctor, and no skincare product is the answer to it, ours included.
What it looks like
The skin turns brown to dark brown or grey-black and feels thickened, like velvet or fine corduroy. The lines of the skin look deeper. The edge is soft and blends into the surrounding skin. Small skin tags often appear in the same areas.
It favours folds: the back and sides of the neck, the underarms, the groin, the elbow creases, under the breasts. The knuckles can be involved too.
It does not itch much and it does not hurt. It also does not wash or scrub off, which is how many people first realise it is not dirt.
How it differs from ordinary darkening
Friction and shaving leave skin that is darker and still flat and smooth to the touch. The colour is in one place, such as the underarms alone, and it follows the cause. What causes dark underarms and a dark neck cover those.
Acanthosis nigricans has texture. It is slightly raised, it feels different from the skin beside it, and it is usually present in more than one fold on both sides of the body.
The colour comes mostly from the thickened surface layer. There is little extra melanin involved. That is why pigment-targeting creams make so little difference to it.
Why it happens
The commonest cause by far is insulin resistance. When the body's cells respond poorly to insulin, the pancreas makes more of it. High insulin levels act on skin cells and make them multiply faster, and the skin in the folds thickens.
Insulin resistance comes before type 2 diabetes, often by years, and the skin change can be the first visible sign of it. It is common in South Asians and is increasingly seen in children and teenagers.
Other causes are less frequent. Polycystic ovary syndrome, thyroid disorders and other hormonal conditions can produce it. So can some medicines, including high-dose niacin, steroids taken by mouth and some hormonal treatments. Excess weight is strongly linked to it, mainly through insulin.
Rarely, acanthosis nigricans appears suddenly and spreads quickly in an adult, sometimes involving the palms or the mouth and coming with unexplained weight loss. That form can be associated with an internal cancer and needs a doctor promptly.
What a doctor checks
The diagnosis is usually made by looking. The tests are for the cause.
Expect blood tests for fasting glucose and long-term blood sugar, sometimes insulin and cholesterol, and thyroid function. Women with irregular periods may be assessed for polycystic ovary syndrome. The doctor will ask about medicines and family history.
A general physician can start this. A dermatologist or an endocrinologist may follow, depending on what the tests show.
What helps
The skin follows the cause. When insulin levels come down, through changes in diet, regular activity, weight loss where it applies, or medicines a doctor prescribes for insulin resistance, the patches usually soften and lighten over the following months. They may not clear completely.
If a medicine is responsible, the prescriber may be able to change it. Do not stop a prescribed medicine without asking.
Doctors sometimes prescribe creams to smooth the thickened surface, such as retinoids or products with urea, lactic acid or salicylic acid. These improve the look and feel. They come second to dealing with the cause, and they are a doctor's decision.
What does not help
Scrubbing does nothing to the thickening and inflames the skin. Bleach and lemon irritate it.
Whitening creams are the worst choice. The fast ones often contain steroids, which thin skin in the folds and leave stretch marks. How to tell whether a cream contains a steroid lists the warning signs, and underarm whitening creams covers the category.
Cosmetic tone-evening products, including niacinamide serums and lotions, are aimed at melanin. This condition is mostly about skin thickness, so they have little to offer here.
Why it is worth acting on
The skin change is harmless. What it points to is not always harmless, and it is far easier to manage early. Insulin resistance found at this stage can often be improved with changes in daily habits, before it becomes diabetes.
For a parent who notices a dark, velvety band on a child's neck, the same applies. It is a reason for a check-up, and scrubbing harder at bath time will not help.
The short version
Dark, velvety, thickened skin in more than one fold is a reason to see a doctor and ask for blood sugar tests. Skincare is not the tool for it.
Cham makes nothing for acanthosis nigricans and would not suggest any of its products for it. The body pigmentation zone map routes this pattern to a doctor for the same reason.
