Saline laxatives are mainly used for cleansing the bowel, in order to prepare for x-rays, tests, or surgery. These products should never be used for long-term treatment. The active ingredient in saline laxatives either magnesium sulfate (Epsom salts) or magnesium citrate. Phillips Milk of Magnesia is a mild saline laxative. These laxatives usually work in about three to five hours.