Your skin is the largest and most exposed organ of your entire body. As such, it is constantly bombarded both internally and externally by factors that can change its structure, texture and color. On the shoulders and arms, these can manifest through areas of hyperpigmentation that create dark spots. These spots can form for several reasons. It is important to remember to see your doctor about spots on your skin to ensure that they are not potentially harmful, such as with skin cancer.
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Unprotected exposure to sunlight is one of the most common reasons spots can develop on your shoulders and arms. As you spend excess time in the sun, your skin becomes altered, causing the melanocytes of your skin tissues to create excess melanin. Melanin is the substance that gives your skin its color. This melanin can clump together, forming hyperpigmented areas of skin called sun spots. Although the spots themselves are not dangerous, sun spots can increase your risk of cancer or may be an early warning sign of skin cancer, states the National Institutes of Health's Medline Plus.
Certain skin injuries can also alter your skin, creating spots on the shoulders and arms as a side effect called post-inflammatory hyperpigmentation. This side effect occurs when excess melanin is left over after the healing process of the skin, such as with a severe acne outbreak, or when old red blood cells die near the skin's surface, leaving an iron pigment behind. In addition to acne, other skin injuries that can cause spots include burns, scrapes, insect bites and cuts.
Certain medications can also cause spots to form on the skin, including on your shoulders and arms. For example, taking estrogen can increase the production of melanin, resulting in blotchy or hyperpigmented skin. Other medications that may cause skin spots as a side effect include the anti-epileptic drug phenytoin, the tranquilizer drugs known as phenothiazines and tetracycline antibiotics.
Disease can also affect the pigmentation of your skin. One of the most prominent of these diseases is the skin cancer known as melanoma, which can develop due to sun damage on the skin. Other diseases and disorders that may cause dark spots include hemachromatosis, Riehl's melanosis, pituitary tumors, Addison's disease and eczema.