Rhubarb is a plant. The root and underground stem (rhizome) are used to make medicine. Rhubarb is used primarily for digestive complaints including constipation, diarrhea,heartburn, stomach pain, gastrointestinal (GI) bleeding, and preparation for certain GI diagnostic procedures. Some people use rhubarb so they have to strain less during bowel movements; this reduces pain from hemorrhoids or tears in the skin lining the anal canal (anal fissures). Rhubarb is sometimes applied to the skin to treat cold sores. In food, rhubarb stems are eaten in pie and other recipes. Rhubarb is also used as a flavoring agent. How does it work? Rhubarb contains several chemicals which might help heal cold sores. information from webmd