The alpha-glucosidase inhibitor acarbose is a drug used to treat type II diabetes mellitus. It occasionally causes diarrhea. Acarbose related colitis has been reported. This note explains how such side effects may occur. Because of small intestine alpha-glucosidase inhibition, increased starch reaches the colon. Increased colonic starch allows the flora to generate increased butyrate. Absorbed butyrate causes up-regulation of prostaglandin E series production and the latter generates water and electrolyte loss. A colitis results when this acarbose driven process is extreme. Acarbose should be avoided in pregnancy until above can be disproved due to teratogenic and labor inducing potential of prostaglandin E. A theoretical reason for avoiding acarbose in Crohn's disease is presented. Despite these considerations, acarbose remains a safe medicine and may even have salutary intestinal consequences stemming from the same physiology as outlined here.