Melanosis coli is the presence of dark pigment deposits in the colon lamina propria that cause dark-brown or black discoloration on the colon lining, notes MedicineNet. Due to the presence of lipofuscin, which is a pigment of destroyed cells, in the deposits, melanosis coli is otherwise called pseudomelanosis coli.
Typically, melanosis coli occurs as a result of a prolonged use of anthranoid laxatives, such as rhubarb derivatives and senna, and does not result in symptoms, according to MedicineNet. When the anthranoid laxatives reach the colon, they transform into active compounds, which damage the colon lining cells. As more cells sustain damaged, the pigmentation occurs on the colon wall.