Jessica Biel doesn't appear to have any problems with underarm darkening. She does, however, seem to have a problem with Adam Sandler sniffing her armpit.

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The armpit. For some, it's an erogenous zone. For others, it's a tickle spot. And for still others -- those individuals who suffer from underarm darkening -- it's a source of embarrassment.

Underarm darkening, or axillary darkening, occurs in men and women, but it's far more common in women, especially women with darker skin. Look into the closet or dresser of one of these women, and you're sure to see a notable absence of sleeveless shirts and bathing suits. Snoop through her medicine cabinet, and you're sure to discover any number of possible treatments, including bleaches and creams. But experimenting with medicines without understanding the causes of underarm darkening can be counterproductive. In some cases, it can be dangerous.

The first order of business is to understand skin pigmentation. Almost everyone experiences changes in skin color from time to time. Just think about a trip to the beach. As ultraviolet (UV) rays strike your exposed skin, they stimulate special cells known as melanocytes to produce a brown-colored pigment. The pigment -- melanin -- protects your skin from UV light, dissipating the harmful energy into heat. If you stay out in the sun a long time, you produce more melanin, and your skin gets darker.

Exposure to sunlight isn't the only thing that can darken skin. Certain diseases and conditions also may play an important role. For example, hyperpigmentation, or excessive skin darkening, is a key symptom of Addison's disease, a disorder that results when the adrenal glands fail to produce sufficient amounts of critical hormones. Hormonal changes during pregnancy also may increase the body's production of melanin, leading to blotchy skin on the upper lip, nose, cheekbones or forehead.

Hyperpigmentation is an example of a pigmentation disorder. Pigmentation disorders occur when skin cells become damaged or unhealthy. This affects their melanin production and, ultimately, the color of the skin. Some pigmentation disorders, like those seen during pregnancy, appear on just patches of skin; others spread over the entire body. One area that can be affected is the underarm.

Unfortunately, Addison's disease and pregnancy are not the only causes of underarm darkening. Several health conditions and hygiene habits can make the dark shadows of the armpits even darker. Let's look at these in greater detail.