Chemotherapy can cause constipation, which is a decreased number of bowel movements and a difficulty passing stools. Constipation may be accompanied by gas or cramping, and can lead to a more serious condition known as stool impaction, in which the stool will no longer move through the digestive tract. Chemotherapy drugs affect your body's ability to move food through the gastrointestinal tract by damaging the cells lining this area. This damage can lead to either diarrhea or constipation, depending on whether this damage increases or decreases intestinal activity.