Rectal sodium phosphate is used to treat constipation that happens from time to time. Rectal sodium phosphate should not be given to children younger than 2 years of age. Rectal sodium phosphate is in a class of medications called saline laxatives. It works by drawing water into the large intestine to produce a soft bowel movement.
Rectal sodium phosphate comes as an enema to insert in the rectum. It is usually inserted when a bowel movement is desired. The enema usually causes a bowel movement within 1 to 5 minutes. Follow the directions on the package label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Use rectal sodium phosphate exactly as directed. Do not use more or less of it or use it more often than directed on the package label. Do not use more than one enema in 24 hours even if you have not had a bowel movement. Using too much rectal sodium phosphate may cause serious damage to the kidneys or heart and possibly death.
Summary from MedlinePlus.gov, click for more information.