Jarrow Formulas Mastic Gum features resin from the mastic tree (Pistacia lentiscus), also called mastiha, a shrub-like tree grown mainly in Greece and Turkey. Composed of a resinous exudate containing essential oils, mastic gum has been used for centuries. Mastic Gum supports the health of the gastric and duodenal cells and tissues to promote gastrointestinal health.