Magnesium citrate is a laxative used to cleanse fecal matter from the bowel prior to surgery or other bowel procedures, such as colonoscopies, according to WebMD. It increases the amount of fluid in the intestine and usually results in a bowel movement within 30 minutes to three hours of ingestion.
When a doctor directs a patient to take magnesium citrate prior to a procedure, he normally advises the patient how long before the procedure to take it, explains WebMD. Dosage is based on individual factors, such as the patient's age and medical condition. Drinking an 8-ounce glass of water after swallowing the magnesium citrate is recommended to keep the body from losing too much water.