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There are at least 4 different reasons you could have dark armpits, such as …

Shaving.

When you shave you cut the hairs off 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.

Remedy: Stop shaving and try waxing or plucking instead so you get rid of the hair below the skin surface.

Buildup of dead skin cells.

According to at least one dermatologist, dark spots under your arms are the result of dead skin cells that are trapped in microscopic “hills and valleys” on your skin.

Remedy: Exfoliate, preferably with a product containing lactic acid.
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.

Remedy: Watch your diet to control insulin production and use Retin-A, 20% urea, alpha hydroxyacids, and salicylic acid prescriptions to lighten your armpits.

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.

Remedy: 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 : http://www.nigeriafilms.com/news/37202/20/photos-4-causes-and-remedies-of-dark-armpits.html