Melanosis coli means that a black pigment has infiltrated the colon wall. It is almost always due to laxatives such as cascara or senna. Melanosis coli is generally benign but in severe cases it may reduce bowel function making constipation worse. Image 1 shows a mild case of melanosis coli with some black speckling across the lining of the colon. In advanced cases as in Image 2, the inside lining becomes pitch black rather than the normal light pink.