Saline Agents:
Contain a cation (magnesium) or anion (sulfate or phosphate) that carries obligatory water of hydration and is poorly absorbed from the intestinal lumen;
Retain fluid in the bowel to promote flow. Examples include magnesium hydroxide (Milk of Magnesia), sodium phosphate and sodium sulfate;
Disadvantage is rapid delivery of a large pressure head to the distal colon and anal sphincter - difficult to time and control.