Dietary Supplement For Stomach Discomfort 500 mg Mastic gum, a natural resin from the Pistacia lentiscus tree of Greece, has been used traditionally by Mediterranean cultures to support gastrointestinal health. Source Naturals Mastic Gum Extract is standardized to 35% masticonic acids, which are the active compounds shown to support the lining of the stomach and the duodenum, and the body's own response to imbalances.