What causes underarm darkening?

Jessica Biel's underarm
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.
Scott Gries/Getty Images

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.


Dark Shadows I: Insulin and Irritated Underarms

Woman applying deodarant
That antiperspirant may help you smell like fresh linen, but it also might irritate your underarms.
Altrendo Images/Getty Images

We've already mentioned how hormonal changes related to pregnancy and Addison's disease can increase melanin production. But there's another common hormone disorder that can lead to underarm darkening. The disorder is diabetes, and the hormone is insulin, the chemical that helps to keep blood glucose in the normal range. In people with type 2 diabetes, cells become resistant to insulin over time. As a result, insulin remains in the bloodstream, and insulin levels become elevated. Normal insulin levels are about 5 to 20 micro units per milliliter while fasting [source: Rennert]. People with insulin resistance can have levels as much as four times higher that.

You probably know that elevated insulin levels can increase the risk for kidney failure, blindness and heart disease. But did you know that too much insulin can overstimulate skin cells? This leads to thickening and darkening of the skin in certain areas, including under the arms. In severe cases, the skin can take on a thick, velvety appearance. Dermatologists refer to this condition as acanthosis nigricans, and they see it most often in people of African descent.


If you have acanthosis nigricans related to diabetes, you should treat the diabetes first. As you bring the underlying hormonal disorder under control, you should see the dark patches fade. If your underarm skin remains dark, try applying over-the-counter topical creams containing lactic acid or glycolic acid, which can help to exfoliate the dark patches.

Another cause of underarm darkening, especially in women with brown skin, relates to external assaults on the skin. For example, chemicals in underarm products, such as deodorants and antiperspirants, can produce an allergic or irritated reaction. Excessive rubbing can have a similar effect. Both situations can lead to hyperkeratosis, a thickening of the skin that's meant to provide protection against rubbing, pressure and irritation. Unfortunately, hyperpigmentation often accompanies hyperkeratosis.

Dermatologists may treat this condition, known as post-inflammatory hyperpigmentation, with prescription creams or lotions. Many of these lotions contain tretinoin, the drug found in the acne medication Retin-A. Other treatment options include oral medications, such as isotretinoin, and dermabrasion, which may help to reduce the thickness of the affected skin.

Other, less serious conditions can result in underarm darkening. We'll look at those next.


Dark Shadows II: Bacteria and Body Hair

Human skin is alive with a host of microorganisms. One recent study published in the journal Science found that at least 18 different phyla of bacteria dwell in 20 different skin habitats. The underarm is one of those habitats. There, in the dark, swampy environment, the bacterium Corynebacterium minutissimum makes a nice home. Normally, it goes unnoticed, but if your body chemistry changes -- if you sweat -- the bug can cause problems. The least serious problem is body odor, which results when the bacteria break down sweat. A more serious problem is erythrasma, a chronic infection in the deeper layers of the skin. Sometimes, the infection can lead to the formation of red, scaly patches. The border of the affected skin is well-defined, with a clear demarcation between the discolored patch and the surrounding normal skin. Over time, the pink or red skin fades to brown.

Doctors can diagnose erythrasma by shining a Wood's lamp on the affected area. A Wood's lamp produces ultraviolet light, which can be used to detect several conditions affecting the skin. If you have the infection, the patches will fluoresce in the presence of the UV light and glow a magenta color. The treatment, as with any bacterial infection, requires a course of either topical or oral antibiotics.


If you have underarm darkening and all other hormonal and bacterial causes have been eliminated, you might evaluate your shaving habits. Underarm hair tends to be dark and coarse, which means that stubble -- the hair left just beneath the surface of the skin after shaving -- is darker and coarser than stubble on your legs. This stubble can be visible under the surface of the skin, especially if there are ingrown hairs. Over time, your underarm skin can look dark and shadowy, much as a man's face may appear to have a "five o'clock shadow" just a few hours after shaving.

Waxing your armpits can help you avoid this problem. Waxing removes hair by the root and leaves no stubble. But the technique hurts, especially in the nerve-rich skin under your arm. It's also more expensive, especially if you go to a salon. A good compromise is to get your first waxing treatment done in a salon to watch how the professionals do it. After that, you can do your own hair removal with waxing kits available in drugstores.

Of course, always consult your doctor if your underarm darkening persists or seems to be related to other symptoms, such as itching or tenderness. Even the lowly armpit can be an effective warning sign of larger, more serious problems.

Keep reading for more links on skin care and skin disorders.


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  • American Academy of Dermatology. "Skin of Color." Consumer health pamphlet, revised 2005. (Sept. 29, 2009) http://www.aad.org/public/publications/pamphlets/general_skin.html
  • Campen, Rebecca. "Prevention best medicine for post-inflammatory hyperpigmentation." The Baltimore Sun. June 5, 2009. (Sept. 29, 2009) http://www.baltimoresun.com/health/experts/sns-health-ask-harvard-hyperpigmentation,0,6094250.story
  • Handwerk, Brian. "Armpits Are 'Rain Forests' for Bacteria, Skin Map Shows." National Geographic News. May 28, 2009. (Sept. 29, 2009) http://news.nationalgeographic.com/news/2009/05/090528-armpits-bacteria-rainforests.html
  • Rennert, Nancy J. "Insulin test." MedlinePlus Medical Encyclopedia. Dec. 6, 2007. (Sept. 29, 2009) http://www.nlm.nih.gov/medlineplus/ency/article/003700.htm
  • Taylor, Susan. "Post-Inflammatory Hyperpigmentation." Brownskin.net. (Sept. 29, 2009) http://www.brownskin.net/hyperpigmentation.html
  • Wu, Jessica. "How Can I Treat Dark Underarm Skin?" Everyday Health. Feb. 20, 2008. (Sept. 29, 2009) http://www.everydayhealth.com/skin-beauty/skin-conditions/specialist/felderman/dark-underarm-skin.aspx