Melanosis is a condition in which the skin or other tissues are abnormally darkened by excessive deposits of the pigment melanin. This hyperpigmentation is also known as melanism and is caused by a disorder in melanin production or metabolism the body. The disorder occurs in humans and other animals.

Found in most living things, melanin is the pigment that determines skin color in humans. People who have higher levels of melanin have darker skin. Freckles and moles are the result of localized melanin concentrations in the skin. Melanin is also present in human hair and eyes.

Melanosis occurs when too much melanin is produced and deposited in the tissues of the body. Common types of this condition are melanosis coli, smoker’s melanosis and ocular melanosis. With the exception of ocular melanosis, most types are benign conditions, and many correct themselves when causal factors are removed.

When dark pigment deposits on the lining of the colon or large intestine cause a brown or black discoloration, it results in melanosis coli. The condition is a bit of a misnomer because there is no melanin in this particular pigment. Melanosis coli, which has no known connection with colon cancer, is commonly caused by the chronic overuse of laxatives. It has no symptoms and usually is observed or diagnosed during a colonoscopy or sigmoidoscopy. When a person stops using laxatives, the discoloration of the colon will lessen and eventually disappear over time.

Smoker’s melanosis is a hyperpigmantation found in the mouth. It is generally associated with smoking but can be found in some non-smokers, it appears as irregular dark patches in the tissues underlying the mucous membranes of the mouth. The discolorations in the mouth will almost always disappear when the person stops smoking. Smoker’s melanosis is benign and is not a precursor of mouth cancer.

Ocular melanosis is also known as melanosis oculi and affects approximately one out of every 5,000 people. This is a congenital condition that is a known risk factor for a cancer of the eye called uveal melanoma. Ocular melanosis can also cause a condition called pigment dispersion syndrome, in which fluid cannot properly drain from the eye. The increased fluid results in greater pressure in the eye and can lead to glaucoma. Ocular melanosis should be monitored closely by an ophthalmologist.

Oral melanosis is the development of dark spots inside the mouth. The spots are similar to those resulting from smoker’s melanosis but are not caused by smoking. It is most common in dark-skinned peoples.

The development of dark patches on the skin or other tissues should always be examined by a physician. He or she will be able to determine whether there is cause for concern. If the diagnosis is melanosis or melanism, the condition probably is harmless.