Did you know that wearing antiperspirant, tight clothing or plucking hairs might be causing your underarm skin to darken? That irritation may be the cause of these dark or splotchy spots, an increase of melanin production in the skin known as hyperpigmentation, although hormone fluctuations and sun damage are also sometimes to blame. The condition appears to affect dark-skinned women most frequently (although no skin color is exempt).
Skin-lightening treatments, such as topical bleaching agents, can be used to treat mild hyperpigmentation problems. Chemical peels can also lighten skin, though recovery time associated with the procedure can range from a couple of days to a couple of weeks. Other risks involved included allergic reaction, scarring and infection [source: WebMD].