What can cause dark pigment under the arms?

The underarms are warm skin folds exposed to rubbing, sweat, deodorant, hair removal, and occasional rashes. Repeated irritation can leave darker colour after the original redness has settled. Atopic dermatitis may cause itch, dryness, and inflammation before pigment remains. Acanthosis nigricans can cause broader, thicker, velvety darkening in the underarms and other folds. The appearance is not enough to identify one cause, so note whether the area is itchy, scaly, sore, lumpy, or changing quickly.

Pause fragranced deodorants and products that sting, and wash with a gentle cleanser rather than scrubbing. Give irritated skin time to settle before shaving or waxing; use a clean, sharp razor and a bland moisturiser if shaving resumes. Wear breathable clothing to limit rubbing. Do not apply unprescribed lightening mixtures or steroid creams. A clinician can treat active dermatitis first and then discuss pigment-focused care when appropriate. If hollows or shadows elsewhere concern you, they are separate issues and need separate assessment rather than the same product.

Arrange an examination for darkening that persists after irritation stops, spreads beyond the underarms, becomes thick or velvety, or returns repeatedly. Seek prompt care for severe pain, pus, open sores, fever, a rapidly enlarging lump, or a sudden widespread skin change. Recurrent rashes may need a treatment plan rather than repeated product changes.

Is this post-inflammatory pigment, dermatitis, or another condition? Could my deodorant or hair-removal method be contributing? What should I use while the skin heals? Is any testing needed because the skin is thickened or changing in other folds?