Laxatives can be used occasionally, but there is some evidence that prolonged overuse can actually damage the colon and worsen constipation. A few medical studies have suggested that longterm use actually damages the nerves in the colon. These include Milk of Magnesia, Duphalac, Colyte, etc. They are generally safe, even when used chronically. However, some patients can be harmed by these.
Examples are cascara and senna.
Both of these substances are stimulant laxatives and can potentially damage the colon. They should be used in moderation just as all laxatives should be. For instance, some peoples' colon works just fine but they can't expel the stool due to incoordination of the pelvic muscles and the rectum. In these cases biofeedback therapy is fast and effective. A special X-ray called a defogram can often show these abnormalities.