Has the area been irritated?

Shaving, tight clothing and reactions to underarm products can contribute to darkening. A rash can leave postinflammatory hyperpigmentation: extra pigment that remains where skin was inflamed or injured, even after the original irritation has healed. Inflammation increases production of melanin, the pigment that gives skin its colour. These marks range from light brown to black. They can affect anyone, but tend to be darker and last longer in darker skin tones. A history of irritation in the same area is a useful clue, rather than a diagnosis by itself.134

Does the skin feel thick or velvety?

Acanthosis nigricans usually develops gradually in skin folds, especially the underarms, neck and groin. Patches are darker and thicker than the surrounding skin, often with a velvety feel. Skin tags can appear on them, and sometimes they itch. The condition is not contagious, and vigorous scrubbing will not remove it. Colour alone cannot distinguish it from pigment left after irritation; texture, where the patches appear and your medical history help a clinician identify the cause.245

Do dark underarms mean diabetes?

Not necessarily. Acanthosis nigricans is often associated with insulin resistance, when the body responds less effectively to insulin, and can signal prediabetes or diabetes. It is also associated with obesity, polycystic ovary syndrome and some medicines, including oral steroids and contraceptives. Healthy people can have it too, particularly people with darker skin, so the patch does not diagnose a blood sugar problem. A clinician can usually recognise the condition by examining your skin and can arrange blood sugar testing when needed. Mention your medicines, family history of diabetes and any other skin changes at the appointment.456

What should I change at home?

If products seem to be irritating the area, reduce how many you apply and reconsider your deodorant. Look for products intended for sensitive skin, without fragrance or dyes. Natural products can still contain substances that irritate skin. Avoid repeatedly exfoliating an irritated area: excessive exfoliation can cause further irritation. If changing deodorant does not settle an itchy, inflamed rash, see a doctor rather than adding a lightening treatment.1

Will the darkness fade?

Pigment left after inflammation is temporary, but can persist, especially in darker skin. Acanthosis nigricans often improves when its underlying cause is treated; there is no fixed fading timetable. Skin lightening products address appearance, which is a smaller job than their packaging sometimes suggests. A dermatologist can consider treatments such as hydroquinone, tretinoin or ammonium lactate after identifying the cause. Peels and lasers are not automatically better: procedures that injure skin can worsen postinflammatory pigmentation.236

When should I get it checked?

Book an appointment for new dark patches, thick or velvety skin, or changes you cannot explain. Seek prompt assessment if thick, dark patches develop rapidly or spread widely: rarely, this pattern is associated with cancer. If the darkness is a distinct mole or mark rather than a broad patch, get it checked if it changes size, shape or colour, bleeds, becomes crusty, or remains an unusual new mark after a few weeks.267

Where this answer comes from

  1. 1Facebookhealth.clevelandclinic.org
  2. 2Acanthosis nigricansnhs.uk
  3. 3Postinflammatory hyperpigmentationdermnetnz.org
  4. 4Acanthosis Nigricans: Causes, Diagnosis, and Treatmentdermnetnz.org
  5. 5Acanthosis nigricans: Overviewaad.org
  6. 6Acanthosis nigricans: MedlinePlus Medical Encyclopediamedlineplus.gov
  7. 7Molesnhs.uk