 Do not give Linzess to children less than two. It may harm them. Do not take Linzess if you have a bowel blockage. Get immediate help if you develop unusual or severe stomach pain, especially with bloody or black stools. The most common side effect is diarrhea, sometimes severe. If it's severe, stop taking Linzess and call your doctor right away. Other side effects include gas, stomach area pain, and swelling. Learn more at Linzess.com or call 1-800-LIN-ZESS.