How often do gastrointestinal carcinoid tumors cause carcinoid syndrome?

Carcinoid syndrome develops in 10 percent or less of carcinoid tumor cases. In this syndrome, the excess hormones produced by carcinoid tumors cause symptoms including flushing, wheezing, diarrhea, intestinal cramps, swelling of the legs and feet, disinterest in sex, and in men, erectile dysfunction. The intestine may lose its ability to absorb nutrients from food. Carcinoid syndrome occurs most often when the gastrointestinal carcinoid tumors have spread from the digestive tract to the liver.