Treating women with severe irritable bowel syndrome (IBS) accompanied by severe diarrhea (usually lasting for 6 months or more). It is only prescribed to women who do not respond to other medicines and is not to be used by women whose main IBS problem is constipation. Certain stomach or intestinal disorders must be ruled out before alosetron is prescribed. Alosetron is a selective 5-HT3 receptor antagonist. It works by blocking the action of serotonin on the intestinal system. This reduces the cramping, stomach pain, stomach discomfort, urgency, and diarrhea caused by IBS.