The following are the most common reasons for dark underarms,

1. Acanthosis nigricans
2. Post-inflammatory hyperpigmentation
3. Friction

Acanthosis nigricans 

The most common causes of Acanthosis nigricans are diabetes, pituitary gland disorder, hypothyroidism and certain forms of cancer. This skin condition causes a dark discolouration in the creases and folds of the body. The darkening and thickening of the skin usually happen in the armpits, nape, under the breasts, navel and groin. While some inherit it genetically, it can happen to any individual.  Birth control medication, thyroid, HGH and steroids can also contribute.

Insulin is a hormone produced by the pancreas to control the amount of glucose in the bloodstream. When the cells offer resistance to insulin and cannot take it up anymore, the abundance of insulin leads to the darkening and thickening of the skin around the folds and creases of the body. Should this happen to you, consult your doctor immediately. They will run a few tests and figure out a course of action. When cancer is not the root cause for Acanthosis nigricans, it will go away when the underlying condition is treated.

Post-inflammatory hyperpigmentation

It is natural for wounds, boils, rashes and chemical burns to heal with pigmentation. 

How to prevent post-inflammatory hyperpigmentation?

1. Waxing and hair removal creams injure the skin. If your underarms are turning dark, avoid these two methods. Instead, shave or trim the underarm hair or if it is affordable, go for laser hair removal. When shaving, don't shave too close.
2. Avoid deodorants and antiperspirants that contain baking soda, alcohol and fragrance of any kind. Simply avoid anything that irritates or stings the skin. 

How to treat post-inflammatory hyperpigmentation?

Use a cream or peel that contains glycolic acid (6% to 12%) or niacinamide on alternate days. Do a small patch test and observe for 24-48 hours before putting it to full use. Moisturize your underarm with an unscented body lotion, then wear the glycolic acid or niacinamide cream and then apply a baking soda and alcohol-free deodorant or a fragrance-free deodorant. Super Deodorant is a good fit here. For peels, follow the instruction on the label.

If you want us to make this cream or peel, please let us know in the comments below or by email.

Friction

Friction happens when two surfaces rub against each other. Obesity, overweight, sagging skin and tight fitted clothing, all-cause friction.  This can lead to injury in the form of redness and irritation. Injuries naturally heal with pigmentation. 

How to prevent friction?

Keep your underarm injury-free. Moisturize your underarms well and choose clothes with a wider arm hold. Don't try to scrub it off in the shower. It will only aggravate the problem.