In this video, Mark Yoder (N.P., Oncology) describes constipation and digestion issues related to cancer treatment, and some tips for dealing with this common side effect. When you have difficulty moving your bowels or move them less than 3 times per week, you are experiencing constipation. If you are constipated for long periods of time, you may develop other problems, so it is imperative to contact your doctor immediately. Chemotherapy drugs and pain medication are well known causes of constipation. Any kind of intestinal surgery as well as many cancer-fighting drugs can weaken the muscles which move the bowels, leading to constipation. Additionally, when you do not eat or drink for a few days, constipation will follow.