It’s not surprising that so many people have this problem, there are at least FIVE different reasons your pits could be darker than they should be.
Cause #1: Shaving.
When you shave you cut the hairs off at, or just below, the surface of the skin. If your hairs are slightly darker than your skin color, they can give the appearance that your skin has a dark stain when it`s really just sub-surface hair.
What To Do About It: Stop shaving and try waxing or plucking instead so you get rid of the hair below the skin surface. Since the hairs aren’t lurking so close to the top of your skin, they won’t be as visible.
Cause #2: Buildup of dead skin cells.
According to at least one dermatolgist, dark spots under your arms are the result of dead skin cells that are trapped in microscopic “hills and valleys” on your skin.
What To Do About It: Exfoliate, preferably with a product containing lactic acid.
Cause #3: Antiperspirant and deodorant usage. In theory, some ingredients in these products (perhaps the fragrance) could be reacting with the skin to cause discoloration. Practically speaking this seems unlikely but many people do claim that when they stop using APDs, the darkness goes away.
What To Do About It: Try switching brands or using a deodorant instead of an antiperspirant. You may stink a bit more, but hey, the Left Brain lives for experiments like that!
Cause #4: A medical condition called
acanthosis nigricans .
This condition causes light-brown-to-black markings on the neck, under the arms, or in the groin. It can be related to insulin production or to a glandular disorder and it typically occurs in people who are overweight.
What To Do About It: Watch your diet to control insulin production and use Retin-A, 20% urea, alpha hydroxyacids, and salicylic acid prescriptions to lighten your armpits.
Cause #5: Hyper-pigmentation.
This condition causes your skin to produce excess melanin pigment. It doesn`t usually affect armpits, so it’s a less likely cause.
What To Do About It: Use a skin bleaching cream to destroy the excess melanin. The Beauty Brains don’t recommend this unless you consult a dermatologist first. You can also try laser treatment to destroy the pigment.
Source: The Beauty Brains