Lasix depletes the body of magnesium, so that increase your risk for deficiency. Magnesium is safe for bowel conditions especially if you choose a highly absorbable form (such as magnesium glycinate or magnesium threonate–discussed in comments above). If it is absorbed quickly into the blood stream, then magnesium will not remain in the bowel to act as an osmotic laxative (to draw water into the bowel). Also, you can start low with the dose and work up. Take directly after food, and if you experience diarrhoea, then split the dose (eg. smaller amount taken twice daily).