Citramag sachets contain magnesium carbonate and citric acid. When these are mixed in water magnesium citrate is formed. Magnesium citrate is the active ingredient of this medicine. It is known as an osmotic laxative. nMagnesium citrate works by causing water to be drawn into the large intestine. This increases the water content and volume of the stools, which stimulates the colon to contract and move its contents along to the rectum, so that the bowel can then be emptied.It causes frequent, liquid stools to be passed until the bowel is emptied. This laxative medicine is used to cleanse the bowel prior to examination or surgical procedures that require the colon to be clean of stools, eg colonoscopy or barium enema.nCitramag is not used to treat constipation.