First described by Andral and Cruveilhier in 1830, Melanosis coli is a condition associated with the deposition of lipofuscin (not melanin as the name might imply) in the lamina propria of the large intestines. Historically, laxatives, primarily from the anthranoid group (i.e., senna and rhubarb derivatives), are the main culprits. As these laxative supplements pass through the colon, they become active and cause cell death and apoptosis in the lining of the colon, eventually causing dark pigmentation of the colon. Diagnosis is typically made by colonoscopy. Discontinuing laxative use usually leads to resolution of melanosis coli. Melanosis coli is not associated with an increased risk of colon cancer.
Chronic laxative use, typically from the anthranoid group, causes Melanosis coli. Senna glycoside (Senna) is the main causative laxative. As the laxative travels down the intestinal tract, it remains in its inactive form until it reaches the large intestine. Once in the large intestine, the molecules transform into their active form, leading to cell damage in the lining of the intestine and death, apoptosis. As these cells undergo apoptosis, they create a dark pigment, lipofuscin, which eventually lines the large bowel. Eventually, these dead cells are taken up by macrophages. Melanosis coli can occur within only a few months of chronic laxative use.
Anyone who abuses laxatives or using them chronically is at risk of developing melanosis coli. It is typically seen in individuals over the age of 45, but it has been reported in younger individuals. The rate increases with age. It is equally common in both sexes. Melanosis coli has also been seen in patients with chronic colitis (IBD) who do not abuse laxatives.
Melanosis coli is caused by chronic laxative use. Anthraquinones in the laxatives have a direct toxic effect on the epithelial cells of the colon. When these laxative molecules turn into their active form in the large bowel, they can cause cell damage to the lining of the large bowel, leading to cell death. As the cells die, they create a dark pigment, which is lipofuscin deposition in the macrophages within the lamina propria, that is visible under colonoscopy. Melanosis coli is more common in the proximal bowel and much rarer in the distal colon. This is probably due to the difference in the macrophages distribution in the colonic was. Once laxative use is halted, the dark pigmentations may resolve.
Under microscopic evaluation, pigment-laden macrophages are typically seen on periodic acid–Schiff stain.
Anthraquinones laxatives work in three ways:
The adverse effects of Senna and other stimulant laxatives include hypovolemia, leading to dizziness, palpitation, syncope; miscarriages in women; and decreased absorption of oral medications due to uptake inhibition.
Typically melanosis coli cause no symptoms and is incidentally found during colonoscopy for other reasons. A detailed history should be focused on chronic laxative use, typically Senna or Cascara which contain anthraquinone, and history of IBD or colitis. These stimulant laxatives also can be found in over-the-counter herbal supplements and teas. Physical findings of melanosis coli are described through colonoscopy as black pigmentations lining the colonic mucosa.
Evaluation of melanosis coli is through colonoscopy. Biopsies should be taken to rule out true melanosis which may relate to more threatening diseases such as Peutz-Jegher Syndrome. No laboratory or imaging studies will help in making the diagnosis of melanosis coli. Characteristically, melanosis coli is most intense at the cecum and diminishes as it moves down the colon. 
Treatment of melanosis coli is to stop laxative use. Resolution takes up to a year. 
It is important to rule other causes before making the diagnosis of melanosis coli. Peutz-Jeghers Syndrome, an autosomal dominant disorder that leads to numerous hamartomatous polyps in the bowel with distinct findings of melanin deposition in the skin and mucosa, may look like melanosis coli. Patients with this disease are at a 15-fold increase of developing colon cancer and must be ruled out prior to making the diagnosis of melanosis coli.
The prognosis is extremely positive in these patients. Melanosis coli will typically go away with causative laxative cessation. This can take up to one year. Rebound constipation can occur with laxative cessation.
Some research has found a correlation between melanosis coli and colonic polyps. However, there is no consensus or evidence relating to an increased risk of colon cancer. There has been research supportive of melanosis coli and colorectal adenomas. Increased stimulant laxative use can cause electrolyte derangements due to the rapid transit through the bowel and lack of nutrition and water reabsorption. The most common is potassium imbalance which may be fatal.
A gastroenterologist is needed to make the diagnosis by colonoscopy and exclude potential alternate causes. Dieticians may also be a benefit in cases of constipation.
It is important to note that laxative use is meant to be a temporary treatment for acute constipation. A healthy bowel regimen is essential for a healthy lifestyle, and one should not depend on stimulant laxatives such as Senna. A diet hardy of fruits, vegetables, whole grains, and legumes will give the essential fiber needed to bulk stool and prevent constipation. Adequate hydration is another key step in having healthy bowel movements. One must keep in mind that other stimulants can cause constipation as well, such as coffee, energy drinks, fried food, excess carbohydrates, alcohol, red meat, and artificial sweeteners. Adequate physical exercise also stimulates the bowels allowing for peristalsis, rhythmic motion of the intestines, to take place and help with the transit of stool down the digestive tract. Adequate sleep helps the body regulate appropriate bowel health.
Melanosis coli, in itself, is not a fatal disease and does not cause any known noticeable disease, but further research is ongoing. Diet alterations and increased physical exercise should be the mainstream and initial treatment for constipation. Oral fiber supplements such as psyllium should be used as a next step. Remember, stimulant laxative use was meant to be a short-term therapy for constipation and should not be used chronically.