I would start with 400mg of magnesium in a capsule and then increase it gradually if need be till you have a bowel movement. Don’t take more than 1500mg for a long period of time though as it can throw off your calcium.

It is unfortunately just causing an osmotic diarrhea situation so it is not a great long term solution.

Try drinking a glass of warm water every morning when you wake up to give the intestines a bath. The head to the water tends to relax the smooth muscle of the bowel.

Be sure to eat plenty of fiber and drink a gallon of water daily and MOVE. Your digestive tract responds to movement. Sitting around all day will result in a sluggish bowel.

http://kitchentablemedicine.com/constipation-dietary-guidelines/