Magnesium citrate is the active ingredient in many over-the-counter laxative preparations for a colonoscopy. It pulls water into the small intestine to assist with cleansing solid matter out of the colon, making it easier for the doctor to see any polyps or lesions, according to Harvard Medical School.
Some physicians ask their patients to use the laxative bisacodyl, branded as Dulcolax, with magnesium citrate the day before the colonoscopy. If the doctor uses a split dose, the patient takes half the magnesium citrate the night before the procedure and the rest six hours ahead of the colonoscopy. However, patients who have kidney disease should use a different type of laxative preparation, according to Harvard Medical School.
A laxative preparation is only part of the process of clearing out the bowel for a colonoscopy. Starting a few days ahead of the procedure, patients should start getting the fiber out of their diets by staying away from raw produce, dried fruit, seeds, nuts and whole grains. The day before the colonoscopy, patients should not eat any solid foods. Instead, they should restrict themselves to clear juices, black coffee, tea, clear bouillon or broth, and clear sodas and sports drinks. Popsicles and Jell-O are considered clear liquid foods and are also acceptable, according to Harvard Medical School.