It's the xylitol in them. It's also in sugar free gum but in lower levels. Can be harmful to pets. I took a tour of the Jelly Belly factory in Fairfield, CA once. At the end I bought a small bag if sugar free jelly beans. Ate about 10 and in a couple of hours, it cleaned ne out. Glad I was at home. Looked at the small print on the bag. Said to eat only 2-3 at a time. "May have laxative effect."