 The brown pigment is lipofuscin in macrophages, not melanin.The most common cause of melanosis coli is the extended use of laxatives, and commonly anthraquinone containing laxatives such as senna, aloe vera and other plant glycosides.The anthranoid laxatives pass through the gastrointestinal tract unabsorbed until they reach the large intestine, where they are changed into their active forms.